diff --git a/.circleci/config.yml b/.circleci/config.yml
index a36a2c3bd..4887a5051 100644
--- a/.circleci/config.yml
+++ b/.circleci/config.yml
@@ -56,7 +56,14 @@ jobs:
       - checkout
       - attach_workspace:
           at: .
-      - run: npm run build -- --gdevelop-root-path GDevelop
+      # Off `main`, the build is deployed to staging: it uses the themes of the
+      # staging assets for the starters re-skinned with each theme.
+      - run: |
+          if [ "$CIRCLE_BRANCH" = "main" ]; then
+            npm run build -- --gdevelop-root-path GDevelop
+          else
+            npm run build -- --gdevelop-root-path GDevelop --staging
+          fi
       - run: npm run check-post-build
       - persist_to_workspace:
           root: .
@@ -71,6 +78,23 @@ jobs:
           at: .
       - aws-cli/setup
       - run: npm run deploy -- --cf-zoneid $CLOUDFLARE_ZONE_ID --cf-token $CLOUDFLARE_TOKEN
+  deploy-staging:
+    docker:
+      - image: cimg/node:20.13.1
+    steps:
+      - run:
+          # A pull request from a fork has no access to the credentials.
+          name: Skip if the credentials are not available
+          command: |
+            if [ -z "$AWS_ACCESS_KEY_ID" ]; then
+              echo "No AWS credentials (pull request from a fork?): not deploying to staging."
+              circleci-agent step halt
+            fi
+      - checkout
+      - attach_workspace:
+          at: .
+      - aws-cli/setup
+      - run: npm run deploy -- --staging --cf-zoneid $CLOUDFLARE_ZONE_ID --cf-token $CLOUDFLARE_TOKEN
 
   # Run the gameplay tests of the example games with a real GDevelop: the
   # latest Linux build published on S3 by GDevelop's own CI is downloaded and
@@ -201,6 +225,38 @@ workflows:
           filters:
             branches:
               only: main
+      - deploy-staging:
+          requires:
+            - build
+            - tests
+          filters:
+            branches:
+              ignore: main
+
+  # Every night (03:00 UTC): rebuild and redeploy `main`. The starters re-skinned
+  # with each theme are generated by the build from the themes and assets the
+  # assets repository publishes, so this is how they catch up with it. Only this
+  # workflow runs on the schedule: the gameplay tests are not run again, as the
+  # games did not change.
+  nightly-rebuild:
+    triggers:
+      - schedule:
+          cron: '0 3 * * *'
+          filters:
+            branches:
+              only: main
+    jobs:
+      - install
+      - build:
+          requires:
+            - install
+      - tests:
+          requires:
+            - install
+      - deploy:
+          requires:
+            - build
+            - tests
 
   # On `main` (or when explicitly asked for): check that every gameplay test
   # of the repository still passes.
diff --git a/README.md b/README.md
index 218a0058e..cb4f96276 100644
--- a/README.md
+++ b/README.md
@@ -35,6 +35,107 @@ If you know how to create _Pull Requests_, you can also clone this repository an
 
 To add a game to the homepage the game have to be listed in the `scripts/generate-database.js` file.
 
+### Theme slots
+
+A *theme* replaces the placeholder art of a 3D starter with a coherent set of
+assets, so a game created by the AI looks like the setting the user asked for.
+Themes and starters never refer to each other: both point at *slots*, roles
+such as `character.player` or `env.ground`.
+
+- `theme-slots.json`, at the root, is the list of slots. It is the vocabulary
+  shared by every starter and every theme.
+- `examples/<starter>/theme-slots.json` says which objects of that starter play
+  which slot:
+
+```json
+{
+  "version": 1,
+  "objects": {
+    "scene:Game Scene/Player": "character.player",
+    "scene:Game Scene/Ground": "env.ground",
+    "scene:Game Scene/Crate": { "top": "env.ground", "front": "env.wall" },
+    "object:TankConfiguration::CombinedTank/TankBase": "vehicle.tank.base"
+  },
+  "effects": {
+    "effect:Game Scene//SkyBox": { "frontFaceResourceName": "sky.day.front" }
+  },
+  "ignoredObjects": ["scene:Game Scene/Camera"]
+}
+```
+
+An object is referred to as `scene:<scene>/<object>`, `global/<object>`, or
+`object:<Extension>::<CustomObject>/<child>` for the children of a custom
+object. A 3D model maps to one model slot. A 3D cube maps either to one texture
+slot for all its faces, or to one slot per face. A skybox effect maps each of
+its texture parameters.
+
+**When you add a 3D starter, add its `theme-slots.json`.** The build refuses to
+publish a 3D starter without one, or one where a 3D model or cube is neither
+mapped nor listed in `ignoredObjects`, and it names the object:
+
+```
+Starter "starting-3d-sailing": the Scene3D::Model3DObject "scene:Game Scene/Boat"
+is neither mapped to a slot nor listed in ignoredObjects in theme-slots.json.
+```
+
+- **Reuse an existing slot whenever the object plays an existing role.** A new
+  starter's hero is `character.player`. Every theme then covers it already.
+- **Add a slot to the root `theme-slots.json` only for a genuinely new role.**
+  Every theme is missing it until someone fills it in the assets repository:
+  those objects keep their placeholder art in the meantime, which is not an
+  error.
+- **List in `ignoredObjects`** what is not meant to be seen as art: the camera
+  anchor cube, an invisible collision helper, the faceless player of a first
+  person game.
+
+Renaming or removing an object makes the build fail the same way on the entry
+that no longer matches: fix or delete the line it names. HUD sprites, fonts and
+sounds are not part of this.
+
+#### Themed starters
+
+The build uses these mappings to write, next to each 3D starter, a copy of it
+re-skinned with each theme published by the assets repository:
+
+```
+examples/starting-3d-tank/starting-3d-tank.json                # the starter
+examples/starting-3d-tank/starting-3d-tank.theme-pirate.json   # its pirate copy
+```
+
+When the AI picks a theme, GDevelop opens the copy instead of the starter, so
+the game shows the themed assets from the first frame. The copies live in the
+starter's folder because a starter refers to its images, sounds and fonts by a
+relative path. They are not examples of their own and are never listed:
+`themedStarters.json`, in the database, says which themes exist and which
+starters each one covers. It is also what the AI prompts read, so a theme is
+only offered once its copies are there.
+
+A theme only re-skins what it has an asset for. Anything else keeps the
+starter's placeholder art.
+
+The copies are rebuilt on every deploy of this repository, and every night by
+the `nightly-rebuild` workflow of the CircleCI configuration, which is how they
+catch up with a theme or an asset published by the assets repository. A new
+theme is therefore available the day after it is merged there (or right away,
+by pushing to `main` here or rerunning the last `main` pipeline in CircleCI).
+
+#### Trying a theme before merging
+
+Any branch but `main` is built with `--staging` and deployed next to the live
+examples, under `staging/examples` and `staging/examples-database`, like the
+assets repository does. A staging build uses the themes of the **staging**
+assets, so a theme pushed on a branch of the assets repository can be tried end
+to end without merging anything:
+
+1. push the theme on a branch of the assets repository (deployed to staging);
+2. push (or rerun the pipeline of) a branch here, so the starters re-skinned
+   with it are built and deployed to staging;
+3. push a branch of the AI prompts, whose dev prompts list the staging themes;
+4. in a development build of GDevelop, turn on "Show staging assets" in the
+   asset store, then create a game with the AI.
+
+Staging holds the result of the last branch that was pushed, whoever pushed it.
+
 ### Gameplay tests
 
 A game can contain _gameplay tests_: scripts that play the game like a player
diff --git a/examples/starting-3D-platformer/theme-slots.json b/examples/starting-3D-platformer/theme-slots.json
new file mode 100644
index 000000000..c788044f7
--- /dev/null
+++ b/examples/starting-3D-platformer/theme-slots.json
@@ -0,0 +1,38 @@
+{
+  "version": 1,
+  "objects": {
+    "scene:Game Scene/Ground": {
+      "front": "env.ground",
+      "left": "env.wall",
+      "right": "env.wall",
+      "top": "env.wall",
+      "bottom": "env.wall"
+    },
+    "scene:Game Scene/Player": "character.player",
+    "scene:Game Scene/Obstacle": {
+      "front": "env.wall",
+      "left": "env.wall",
+      "right": "env.wall",
+      "top": "env.wall",
+      "bottom": "env.wall"
+    },
+    "scene:Game Scene/Coin": "pickup.coin",
+    "scene:Game Scene/PushableBox": {
+      "front": "env.crate",
+      "left": "env.crate",
+      "right": "env.crate",
+      "top": "env.crate",
+      "bottom": "env.crate"
+    }
+  },
+  "effects": {
+    "effect:Game Scene//SkyBox": {
+      "backFaceResourceName": "sky.day.back",
+      "bottomFaceResourceName": "sky.day.bottom",
+      "frontFaceResourceName": "sky.day.front",
+      "leftFaceResourceName": "sky.day.left",
+      "rightFaceResourceName": "sky.day.right",
+      "topFaceResourceName": "sky.day.top"
+    }
+  }
+}
\ No newline at end of file
diff --git a/examples/starting-3d-car-racing/theme-slots.json b/examples/starting-3d-car-racing/theme-slots.json
new file mode 100644
index 000000000..a56a7542a
--- /dev/null
+++ b/examples/starting-3d-car-racing/theme-slots.json
@@ -0,0 +1,27 @@
+{
+  "version": 1,
+  "objects": {
+    "scene:Game Scene/Road_3D": "env.road",
+    "scene:Game Scene/Grass_3D": "env.ground",
+    "scene:Game Scene/PlayerCar": "vehicle.car",
+    "scene:Game Scene/TrafficCone": "prop.obstacle",
+    "scene:Game Scene/FinishLine_3D": {
+      "front": "env.finish",
+      "left": "env.finish",
+      "right": "env.finish",
+      "top": "env.finish",
+      "bottom": "env.finish"
+    },
+    "scene:Game Scene/Walls_3D": "env.wall"
+  },
+  "effects": {
+    "effect:Game Scene//SkyBox": {
+      "backFaceResourceName": "sky.day.back",
+      "bottomFaceResourceName": "sky.day.bottom",
+      "frontFaceResourceName": "sky.day.front",
+      "leftFaceResourceName": "sky.day.left",
+      "rightFaceResourceName": "sky.day.right",
+      "topFaceResourceName": "sky.day.top"
+    }
+  }
+}
\ No newline at end of file
diff --git a/examples/starting-3d-draggable-tiles/theme-slots.json b/examples/starting-3d-draggable-tiles/theme-slots.json
new file mode 100644
index 000000000..1773e84c5
--- /dev/null
+++ b/examples/starting-3d-draggable-tiles/theme-slots.json
@@ -0,0 +1,17 @@
+{
+  "version": 1,
+  "objects": {
+    "scene:Game Scene/Ground": {
+      "front": "env.ground.tiled"
+    },
+    "scene:Game Scene/Unit": "character.player",
+    "scene:Game Scene/Tower": "building.tower",
+    "scene:Game Scene/Tree": "nature.tree",
+    "scene:Game Scene/PlacementGrid": {
+      "front": "env.tiles"
+    }
+  },
+  "ignoredObjects": [
+    "scene:Game Scene/Camera"
+  ]
+}
\ No newline at end of file
diff --git a/examples/starting-3d-driving/theme-slots.json b/examples/starting-3d-driving/theme-slots.json
new file mode 100644
index 000000000..af6a0aa79
--- /dev/null
+++ b/examples/starting-3d-driving/theme-slots.json
@@ -0,0 +1,20 @@
+{
+  "version": 1,
+  "objects": {
+    "scene:Game Scene/Road_3D": "env.road",
+    "scene:Game Scene/Grass_3D": "env.ground",
+    "scene:Game Scene/PlayerCar": "vehicle.car",
+    "scene:Game Scene/TrafficCone": "prop.obstacle",
+    "scene:Game Scene/Walls_3D": "env.wall"
+  },
+  "effects": {
+    "effect:Game Scene//SkyBox": {
+      "backFaceResourceName": "sky.day.back",
+      "bottomFaceResourceName": "sky.day.bottom",
+      "frontFaceResourceName": "sky.day.front",
+      "leftFaceResourceName": "sky.day.left",
+      "rightFaceResourceName": "sky.day.right",
+      "topFaceResourceName": "sky.day.top"
+    }
+  }
+}
\ No newline at end of file
diff --git a/examples/starting-3d-endless-runner/theme-slots.json b/examples/starting-3d-endless-runner/theme-slots.json
new file mode 100644
index 000000000..a9c686c67
--- /dev/null
+++ b/examples/starting-3d-endless-runner/theme-slots.json
@@ -0,0 +1,22 @@
+{
+  "version": 1,
+  "objects": {
+    "scene:Game Scene/Platform_Ground": {
+      "front": "env.wall",
+      "left": "env.ground",
+      "right": "env.ground",
+      "top": "env.ground"
+    },
+    "scene:Game Scene/Hazard": "hazard.spike",
+    "scene:Game Scene/Platform_Floating": {
+      "front": "env.ground",
+      "left": "env.ground",
+      "right": "env.ground",
+      "top": "env.ground"
+    },
+    "scene:Game Scene/Player": "character.player"
+  },
+  "ignoredObjects": [
+    "scene:Game Scene/Camera"
+  ]
+}
\ No newline at end of file
diff --git a/examples/starting-3d-flight-sim/theme-slots.json b/examples/starting-3d-flight-sim/theme-slots.json
new file mode 100644
index 000000000..dc6b2a517
--- /dev/null
+++ b/examples/starting-3d-flight-sim/theme-slots.json
@@ -0,0 +1,19 @@
+{
+  "version": 1,
+  "objects": {
+    "scene:Game Scene/Ground": {
+      "front": "env.ground.tiled"
+    },
+    "scene:Game Scene/PlayerAircraft": "vehicle.aircraft.player"
+  },
+  "effects": {
+    "effect:Game Scene//SkyBox": {
+      "backFaceResourceName": "sky.day.back",
+      "bottomFaceResourceName": "sky.day.bottom",
+      "frontFaceResourceName": "sky.day.front",
+      "leftFaceResourceName": "sky.day.left",
+      "rightFaceResourceName": "sky.day.right",
+      "topFaceResourceName": "sky.day.top"
+    }
+  }
+}
\ No newline at end of file
diff --git a/examples/starting-3d-point-and-click-adventure/theme-slots.json b/examples/starting-3d-point-and-click-adventure/theme-slots.json
new file mode 100644
index 000000000..8d798ec8d
--- /dev/null
+++ b/examples/starting-3d-point-and-click-adventure/theme-slots.json
@@ -0,0 +1,18 @@
+{
+  "version": 1,
+  "objects": {
+    "scene:Game Scene/Wall": "env.wall",
+    "scene:Game Scene/Water": {
+      "front": "env.water"
+    },
+    "scene:Game Scene/Ground": {
+      "front": "env.ground.tiled"
+    },
+    "scene:Game Scene/Player": "character.player",
+    "scene:Game Scene/NPC": "character.npc",
+    "scene:Game Scene/Grass": "nature.grass"
+  },
+  "ignoredObjects": [
+    "scene:Game Scene/Camera"
+  ]
+}
\ No newline at end of file
diff --git a/examples/starting-3d-rts-unit-selection/theme-slots.json b/examples/starting-3d-rts-unit-selection/theme-slots.json
new file mode 100644
index 000000000..ece943ca4
--- /dev/null
+++ b/examples/starting-3d-rts-unit-selection/theme-slots.json
@@ -0,0 +1,17 @@
+{
+  "version": 1,
+  "objects": {
+    "scene:Game Scene/Ground": {
+      "front": "env.ground.tiled"
+    },
+    "scene:Game Scene/RTSUnit": "character.player",
+    "scene:Game Scene/Building_Impassable": "env.wall",
+    "scene:Game Scene/Water_Passable": {
+      "front": "env.water"
+    },
+    "scene:Game Scene/Grass": "nature.grass"
+  },
+  "ignoredObjects": [
+    "scene:Game Scene/Camera"
+  ]
+}
\ No newline at end of file
diff --git a/examples/starting-3d-shootemup/theme-slots.json b/examples/starting-3d-shootemup/theme-slots.json
new file mode 100644
index 000000000..8c51275f9
--- /dev/null
+++ b/examples/starting-3d-shootemup/theme-slots.json
@@ -0,0 +1,15 @@
+{
+  "version": 1,
+  "objects": {
+    "scene:Game Scene/RepeatingBackground": {
+      "front": "env.ground.tiled"
+    },
+    "scene:Game Scene/Player": "vehicle.aircraft.player",
+    "scene:Game Scene/Enemy": "vehicle.aircraft.enemy",
+    "scene:Game Scene/PlayerBullet": "projectile.player",
+    "scene:Game Scene/EnemyBullet": "projectile.enemy"
+  },
+  "ignoredObjects": [
+    "scene:Game Scene/Camera"
+  ]
+}
\ No newline at end of file
diff --git a/examples/starting-3d-tank/theme-slots.json b/examples/starting-3d-tank/theme-slots.json
new file mode 100644
index 000000000..422896d5c
--- /dev/null
+++ b/examples/starting-3d-tank/theme-slots.json
@@ -0,0 +1,23 @@
+{
+  "version": 1,
+  "objects": {
+    "scene:Game Scene/Road_3D": "env.road",
+    "scene:Game Scene/Grass_3D": "env.ground",
+    "scene:Game Scene/Target": "character.enemy",
+    "scene:Game Scene/Bullet": "projectile.player",
+    "scene:Game Scene/Walls_3D": "env.wall",
+    "object:TankConfiguration::CombinedTank/TankBase": "vehicle.tank.base",
+    "object:TankConfiguration::TankTop/TankTop": "vehicle.tank.turret",
+    "object:TankConfiguration::TankTop/TankCanon": "vehicle.tank.cannon"
+  },
+  "effects": {
+    "effect:Game Scene//SkyBox": {
+      "backFaceResourceName": "sky.day.back",
+      "bottomFaceResourceName": "sky.day.bottom",
+      "frontFaceResourceName": "sky.day.front",
+      "leftFaceResourceName": "sky.day.left",
+      "rightFaceResourceName": "sky.day.right",
+      "topFaceResourceName": "sky.day.top"
+    }
+  }
+}
\ No newline at end of file
diff --git a/examples/starting-3d-tile-placement/theme-slots.json b/examples/starting-3d-tile-placement/theme-slots.json
new file mode 100644
index 000000000..1773e84c5
--- /dev/null
+++ b/examples/starting-3d-tile-placement/theme-slots.json
@@ -0,0 +1,17 @@
+{
+  "version": 1,
+  "objects": {
+    "scene:Game Scene/Ground": {
+      "front": "env.ground.tiled"
+    },
+    "scene:Game Scene/Unit": "character.player",
+    "scene:Game Scene/Tower": "building.tower",
+    "scene:Game Scene/Tree": "nature.tree",
+    "scene:Game Scene/PlacementGrid": {
+      "front": "env.tiles"
+    }
+  },
+  "ignoredObjects": [
+    "scene:Game Scene/Camera"
+  ]
+}
\ No newline at end of file
diff --git a/examples/starting-3d-top-down-rpg/theme-slots.json b/examples/starting-3d-top-down-rpg/theme-slots.json
new file mode 100644
index 000000000..1850f26c7
--- /dev/null
+++ b/examples/starting-3d-top-down-rpg/theme-slots.json
@@ -0,0 +1,15 @@
+{
+  "version": 1,
+  "objects": {
+    "scene:Game Scene/Ground": {
+      "front": "env.ground.tiled"
+    },
+    "scene:Game Scene/Player": "character.player",
+    "scene:Game Scene/NPC": "character.npc",
+    "scene:Game Scene/Grass": "nature.grass",
+    "scene:Game Scene/Building_Obstacle": "env.wall"
+  },
+  "ignoredObjects": [
+    "scene:Game Scene/Camera"
+  ]
+}
\ No newline at end of file
diff --git a/examples/starting-3d-twin-stick-shooter/theme-slots.json b/examples/starting-3d-twin-stick-shooter/theme-slots.json
new file mode 100644
index 000000000..d5d92c417
--- /dev/null
+++ b/examples/starting-3d-twin-stick-shooter/theme-slots.json
@@ -0,0 +1,14 @@
+{
+  "version": 1,
+  "objects": {
+    "scene:Game Scene/RepeatingBackground": {
+      "front": "env.ground.tiled"
+    },
+    "scene:Game Scene/Player": "vehicle.aircraft.player",
+    "scene:Game Scene/Enemy": "vehicle.aircraft.enemy",
+    "scene:Game Scene/PlayerBullet": "projectile.player"
+  },
+  "ignoredObjects": [
+    "scene:Game Scene/Camera"
+  ]
+}
\ No newline at end of file
diff --git a/examples/starting-3d-vampire-survivor/theme-slots.json b/examples/starting-3d-vampire-survivor/theme-slots.json
new file mode 100644
index 000000000..0c414f804
--- /dev/null
+++ b/examples/starting-3d-vampire-survivor/theme-slots.json
@@ -0,0 +1,14 @@
+{
+  "version": 1,
+  "objects": {
+    "scene:Game Scene/RepeatingBackground": {
+      "front": "env.ground.tiled"
+    },
+    "scene:Game Scene/Player": "character.player",
+    "scene:Game Scene/Enemy": "character.enemy",
+    "scene:Game Scene/PlayerBullet": "projectile.player"
+  },
+  "ignoredObjects": [
+    "scene:Game Scene/Camera"
+  ]
+}
\ No newline at end of file
diff --git a/examples/starting-first-person-farming/theme-slots.json b/examples/starting-first-person-farming/theme-slots.json
new file mode 100644
index 000000000..362317849
--- /dev/null
+++ b/examples/starting-first-person-farming/theme-slots.json
@@ -0,0 +1,41 @@
+{
+  "version": 1,
+  "objects": {
+    "scene:Game Scene/Ground": {
+      "front": "env.ground"
+    },
+    "scene:Game Scene/DirtPlot": {
+      "front": "env.dirt",
+      "left": "env.dirt",
+      "right": "env.dirt",
+      "top": "env.dirt",
+      "bottom": "env.dirt"
+    },
+    "scene:Game Scene/Carrot": "crop.a",
+    "scene:Game Scene/Beet": "crop.b",
+    "scene:Game Scene/Harvest_Seed_Beet": "prop.container",
+    "scene:Game Scene/Harvest_Seed_Carrot": "prop.container",
+    "scene:Game Scene/Seed_Beet": "nature.rock.small",
+    "scene:Game Scene/Seed_Carrot": "nature.rock.small",
+    "scene:Game Scene/Walls": {
+      "front": "env.wall",
+      "left": "env.wall",
+      "right": "env.wall",
+      "top": "env.wall",
+      "bottom": "env.wall"
+    }
+  },
+  "effects": {
+    "effect:Game Scene//Effect2": {
+      "backFaceResourceName": "sky.day.back",
+      "bottomFaceResourceName": "sky.day.bottom",
+      "frontFaceResourceName": "sky.day.front",
+      "leftFaceResourceName": "sky.day.left",
+      "rightFaceResourceName": "sky.day.right",
+      "topFaceResourceName": "sky.day.top"
+    }
+  },
+  "ignoredObjects": [
+    "scene:Game Scene/Player"
+  ]
+}
diff --git a/examples/starting-first-person-horror/theme-slots.json b/examples/starting-first-person-horror/theme-slots.json
new file mode 100644
index 000000000..924eb36f6
--- /dev/null
+++ b/examples/starting-first-person-horror/theme-slots.json
@@ -0,0 +1,37 @@
+{
+  "version": 1,
+  "objects": {
+    "scene:Game Scene/Ground": {
+      "front": "env.ground",
+      "left": "env.wall",
+      "right": "env.wall",
+      "top": "env.wall",
+      "bottom": "env.wall"
+    },
+    "scene:Game Scene/Obstacle": {
+      "front": "env.wall",
+      "left": "env.wall",
+      "right": "env.wall",
+      "top": "env.wall",
+      "bottom": "env.wall"
+    },
+    "scene:Game Scene/PushableBox": "env.crate",
+    "scene:Game Scene/Monster": "character.monster"
+  },
+  "effects": {
+    "effect:Game Scene//SkyBox": {
+      "backFaceResourceName": "sky.dark.back",
+      "bottomFaceResourceName": "sky.dark.bottom",
+      "frontFaceResourceName": "sky.dark.front",
+      "leftFaceResourceName": "sky.dark.left",
+      "rightFaceResourceName": "sky.dark.right",
+      "topFaceResourceName": "sky.dark.top"
+    }
+  },
+  "ignoredObjects": [
+    "scene:Game Scene/PathfindingObstacle",
+    "scene:Game Scene/Player",
+    "object:Light3D::SpotLight3D/Placeholder",
+    "object:Light3D::PointLight3D/Placeholder"
+  ]
+}
diff --git a/examples/starting-first-person-shooter-horror/theme-slots.json b/examples/starting-first-person-shooter-horror/theme-slots.json
new file mode 100644
index 000000000..fe2281f83
--- /dev/null
+++ b/examples/starting-first-person-shooter-horror/theme-slots.json
@@ -0,0 +1,38 @@
+{
+  "version": 1,
+  "objects": {
+    "scene:Game Scene/Ground": {
+      "front": "env.ground",
+      "left": "env.wall",
+      "right": "env.wall",
+      "top": "env.wall",
+      "bottom": "env.wall"
+    },
+    "scene:Game Scene/Obstacle": {
+      "front": "env.wall",
+      "left": "env.wall",
+      "right": "env.wall",
+      "top": "env.wall",
+      "bottom": "env.wall"
+    },
+    "scene:Game Scene/PushableBox": "env.crate",
+    "scene:Game Scene/Monster": "character.monster",
+    "scene:Game Scene/Gun": "weapon.handheld"
+  },
+  "effects": {
+    "effect:Game Scene//SkyBox": {
+      "backFaceResourceName": "sky.dark.back",
+      "bottomFaceResourceName": "sky.dark.bottom",
+      "frontFaceResourceName": "sky.dark.front",
+      "leftFaceResourceName": "sky.dark.left",
+      "rightFaceResourceName": "sky.dark.right",
+      "topFaceResourceName": "sky.dark.top"
+    }
+  },
+  "ignoredObjects": [
+    "scene:Game Scene/PathfindingObstacle",
+    "scene:Game Scene/Player",
+    "object:Light3D::SpotLight3D/Placeholder",
+    "object:Light3D::PointLight3D/Placeholder"
+  ]
+}
diff --git a/examples/starting-first-person-shooter/theme-slots.json b/examples/starting-first-person-shooter/theme-slots.json
new file mode 100644
index 000000000..d4f52dd2d
--- /dev/null
+++ b/examples/starting-first-person-shooter/theme-slots.json
@@ -0,0 +1,35 @@
+{
+  "version": 1,
+  "objects": {
+    "scene:Game Scene/Ground": {
+      "front": "env.ground",
+      "left": "env.wall",
+      "right": "env.wall",
+      "top": "env.wall",
+      "bottom": "env.wall"
+    },
+    "scene:Game Scene/Obstacle": {
+      "front": "env.wall",
+      "left": "env.wall",
+      "right": "env.wall",
+      "top": "env.wall",
+      "bottom": "env.wall"
+    },
+    "scene:Game Scene/PushableBox": "env.crate",
+    "scene:Game Scene/Gun": "weapon.handheld",
+    "scene:Game Scene/Target": "character.player"
+  },
+  "effects": {
+    "effect:Game Scene//SkyBox": {
+      "backFaceResourceName": "sky.day.back",
+      "bottomFaceResourceName": "sky.day.bottom",
+      "frontFaceResourceName": "sky.day.front",
+      "leftFaceResourceName": "sky.day.left",
+      "rightFaceResourceName": "sky.day.right",
+      "topFaceResourceName": "sky.day.top"
+    }
+  },
+  "ignoredObjects": [
+    "scene:Game Scene/Player"
+  ]
+}
diff --git a/examples/starting-first-person-survival-crafting/theme-slots.json b/examples/starting-first-person-survival-crafting/theme-slots.json
new file mode 100644
index 000000000..216e28b1a
--- /dev/null
+++ b/examples/starting-first-person-survival-crafting/theme-slots.json
@@ -0,0 +1,36 @@
+{
+  "version": 1,
+  "objects": {
+    "scene:Game Scene/Ground": {
+      "front": "env.ground",
+      "left": "env.wall",
+      "right": "env.wall",
+      "top": "env.wall",
+      "bottom": "env.wall"
+    },
+    "scene:Game Scene/Obstacle": {
+      "front": "env.wall",
+      "left": "env.wall",
+      "right": "env.wall",
+      "top": "env.wall",
+      "bottom": "env.wall"
+    },
+    "scene:Game Scene/Log": "resource.log",
+    "scene:Game Scene/Rock": "nature.rock.small",
+    "scene:Game Scene/Harvest_Rock": "nature.rock.big",
+    "scene:Game Scene/Harvest_Tree": "nature.tree"
+  },
+  "effects": {
+    "effect:Game Scene//SkyBox": {
+      "backFaceResourceName": "sky.day.back",
+      "bottomFaceResourceName": "sky.day.bottom",
+      "frontFaceResourceName": "sky.day.front",
+      "leftFaceResourceName": "sky.day.left",
+      "rightFaceResourceName": "sky.day.right",
+      "topFaceResourceName": "sky.day.top"
+    }
+  },
+  "ignoredObjects": [
+    "scene:Game Scene/Player"
+  ]
+}
diff --git a/examples/starting-first-person/theme-slots.json b/examples/starting-first-person/theme-slots.json
new file mode 100644
index 000000000..ba95df2b7
--- /dev/null
+++ b/examples/starting-first-person/theme-slots.json
@@ -0,0 +1,33 @@
+{
+  "version": 1,
+  "objects": {
+    "scene:Game Scene/Ground": {
+      "front": "env.ground",
+      "left": "env.wall",
+      "right": "env.wall",
+      "top": "env.wall",
+      "bottom": "env.wall"
+    },
+    "scene:Game Scene/Obstacle": {
+      "front": "env.wall",
+      "left": "env.wall",
+      "right": "env.wall",
+      "top": "env.wall",
+      "bottom": "env.wall"
+    },
+    "scene:Game Scene/PushableBox": "env.crate"
+  },
+  "effects": {
+    "effect:Game Scene//SkyBox": {
+      "backFaceResourceName": "sky.day.back",
+      "bottomFaceResourceName": "sky.day.bottom",
+      "frontFaceResourceName": "sky.day.front",
+      "leftFaceResourceName": "sky.day.left",
+      "rightFaceResourceName": "sky.day.right",
+      "topFaceResourceName": "sky.day.top"
+    }
+  },
+  "ignoredObjects": [
+    "scene:Game Scene/Player"
+  ]
+}
diff --git a/scripts/deploy.js b/scripts/deploy.js
index a629efe65..71d7bea41 100644
--- a/scripts/deploy.js
+++ b/scripts/deploy.js
@@ -5,8 +5,12 @@ const args = require('minimist')(process.argv.slice(2));
 
 const databasePath = path.join(__dirname, '../dist/database');
 const examplesPath = path.join(__dirname, '../dist/examples');
-const databaseDestination = `s3://resources.gdevelop-app.com/examples-database`;
-const examplesDestination = `s3://resources.gdevelop-app.com/examples`;
+// With `--staging`, deploy next to the live examples without touching them,
+// like the assets repository does. Made from any branch but `main`.
+const staging = args['staging'] !== undefined;
+const prefix = staging ? 'staging/' : '';
+const databaseDestination = `s3://resources.gdevelop-app.com/${prefix}examples-database`;
+const examplesDestination = `s3://resources.gdevelop-app.com/${prefix}examples`;
 
 if (!args['cf-zoneid'] || !args['cf-token']) {
   shell.echo(
@@ -60,8 +64,10 @@ axios
     {
       files: [
         // Update the "database"
-        'https://resources.gdevelop-app.com/examples-database/exampleShortHeaders.json',
-        'https://resources.gdevelop-app.com/examples-database/filters.json',
+        `https://resources.gdevelop-app.com/${prefix}examples-database/exampleShortHeaders.json`,
+        `https://resources.gdevelop-app.com/${prefix}examples-database/filters.json`,
+        `https://resources.gdevelop-app.com/${prefix}examples-database/themeSlots.json`,
+        `https://resources.gdevelop-app.com/${prefix}examples-database/themedStarters.json`,
       ],
     },
     {
diff --git a/scripts/generate-database.js b/scripts/generate-database.js
index ceb781ea1..0e4a46e60 100644
--- a/scripts/generate-database.js
+++ b/scripts/generate-database.js
@@ -2,6 +2,7 @@
 const shell = require('shelljs');
 const path = require('path');
 const fs = require('fs').promises;
+const fsSync = require('fs');
 const { constants } = require('fs');
 const crypto = require('crypto');
 const {
@@ -15,6 +16,15 @@ const allLicenses = require('./lib/licenses.json');
 const args = require('minimist')(process.argv.slice(2));
 const { loadSerializedProject } = require('./lib/LocalProjectOpener');
 const { writeProjectJSONFile } = require('./lib/LocalProjectWriter');
+const {
+  loadThemeSlotsVocabulary,
+  checkStarterThemeSlots,
+} = require('./lib/ThemeSlots');
+const {
+  applyThemeToStarter,
+  getThemedStarterFileName,
+} = require('./lib/StarterThemer');
+const { default: axios } = require('axios');
 
 /** @typedef {import('./lib/FileTreeParser.js').DreeWithMetadata} DreeWithMetadata */
 /** @typedef {import('./types').libGDevelop} libGDevelop */
@@ -672,6 +682,103 @@ const createPlatformExtensionsMap = (gd) => {
   return platformExtensionsMap;
 };
 
+// With `--staging`, the build is meant for the staging deployment (made from
+// any branch but `main`): it uses the themes of the staging assets, so a theme
+// and the starters re-skinned with it can be tried before anything is merged.
+const staging = args['staging'] !== undefined;
+const publicBaseUrl = staging
+  ? 'https://resources.gdevelop-app.com/staging'
+  : 'https://resources.gdevelop-app.com';
+
+// The themes are built and published by the assets repository.
+const THEMES_BASE_URL = `${publicBaseUrl}/assets-database/themes`;
+
+/**
+ * The published themes. No theme published yet is a valid answer (the file is
+ * then missing); any other failure aborts the build, as publishing an empty
+ * list would silently turn the themes off.
+ * @returns {Promise<Array<import('./lib/StarterThemer.js').Theme>>}
+ */
+const fetchThemes = async () => {
+  /** @type {Array<{id: string}>} */
+  let themeShortHeaders = [];
+  try {
+    themeShortHeaders = (await axios.get(`${THEMES_BASE_URL}/themes.json`))
+      .data;
+  } catch (error) {
+    const status = error.response && error.response.status;
+    if (status === 403 || status === 404) {
+      console.info('ℹ️ No theme is published yet: no themed starter to build.');
+      return [];
+    }
+    throw error;
+  }
+
+  return Promise.all(
+    themeShortHeaders.map(
+      async ({ id }) => (await axios.get(`${THEMES_BASE_URL}/${id}.json`)).data
+    )
+  );
+};
+
+/**
+ * Write, next to each 3D starter, a copy of it re-skinned with each theme
+ * (`<slug>.theme-<id>.json`). They sit in the starter's folder so the resources
+ * it refers to by a relative path still resolve, and are not examples of their
+ * own: only `themedStarters.json` lists them.
+ * @param {Object.<string, any>} themeSlotsByStarterSlug
+ * @param {Object.<string, string>} starterFilePathBySlug
+ * @returns {Promise<Array<{id: string, name: string, description: string, starters: Object.<string, string>}>>}
+ */
+const generateThemedStarters = async (
+  themeSlotsByStarterSlug,
+  starterFilePathBySlug
+) => {
+  const themes = await fetchThemes();
+  const themedStarters = [];
+
+  for (const theme of themes) {
+    // Starter slug -> URL of its copy re-skinned with the theme.
+    /** @type {Object.<string, string>} */
+    const starters = {};
+    for (const slug of Object.keys(themeSlotsByStarterSlug)) {
+      const starterFilePath = starterFilePathBySlug[slug];
+      const projectObject = JSON.parse(
+        await fs.readFile(starterFilePath, 'utf8')
+      );
+      const { changedObjectsCount, changedResourcesCount } =
+        applyThemeToStarter(
+          projectObject,
+          themeSlotsByStarterSlug[slug],
+          theme
+        );
+      if (!changedObjectsCount && !changedResourcesCount) continue;
+
+      const themedStarterFilePath = path.join(
+        path.dirname(starterFilePath),
+        getThemedStarterFileName(slug, theme.id)
+      );
+      await fs.writeFile(themedStarterFilePath, JSON.stringify(projectObject));
+      starters[slug] = `${publicBaseUrl}/examples/${normalizePathSeparators(
+        path.relative(examplesRootPath, themedStarterFilePath)
+      )}`;
+    }
+    console.info(
+      `ℹ️ Theme "${theme.id}": ${
+        Object.keys(starters).length
+      } themed starters written.`
+    );
+    themedStarters.push({
+      id: theme.id,
+      name: theme.name,
+      description: theme.description,
+      starters,
+    });
+  }
+
+  return themedStarters;
+};
+
 /**
  * Discover all examples and extract information from them.
  */
@@ -741,6 +848,55 @@ const createPlatformExtensionsMap = (gd) => {
     shell.exit(1);
   }
 
+  // Which objects of each 3D starter a theme re-skins. Checked against the
+  // starter itself so an unmapped object can never ship, then published in
+  // one file for the editor.
+  const themeSlotsVocabulary = await loadThemeSlotsVocabulary();
+  /** @type {Object.<string, any>} */
+  const themeSlotsByStarterSlug = {};
+  /** @type {Object.<string, string>} */
+  const starterFilePathBySlug = {};
+  /** @type {Error[]} */
+  const themeSlotsErrors = [];
+  for (const fileWithMetadata of allExampleFiles) {
+    const slug = path.basename(path.dirname(fileWithMetadata.path));
+    if (!slug.startsWith('starting-') || !fileWithMetadata.parsedContent) {
+      continue;
+    }
+    const themeSlotsPath = path.join(
+      path.dirname(fileWithMetadata.path),
+      'theme-slots.json'
+    );
+    const starterThemeSlots = fsSync.existsSync(themeSlotsPath)
+      ? JSON.parse(await fs.readFile(themeSlotsPath, 'utf8'))
+      : null;
+    themeSlotsErrors.push(
+      ...checkStarterThemeSlots(
+        themeSlotsVocabulary,
+        slug,
+        fileWithMetadata.parsedContent,
+        starterThemeSlots
+      )
+    );
+    if (starterThemeSlots) {
+      themeSlotsByStarterSlug[slug] = starterThemeSlots;
+      starterFilePathBySlug[slug] = fileWithMetadata.path;
+    }
+  }
+  if (themeSlotsErrors.length) {
+    console.error(
+      'There were errors while checking the starter theme slots:',
+      themeSlotsErrors
+    );
+    console.info('Aborting because of these errors.');
+    shell.exit(1);
+  }
+
+  const themedStarters = await generateThemedStarters(
+    themeSlotsByStarterSlug,
+    starterFilePathBySlug
+  );
+
   try {
     shell.mkdir('-p', databaseRootPath);
     shell.mkdir('-p', path.join(databaseRootPath, 'examples'));
@@ -760,6 +916,20 @@ const createPlatformExtensionsMap = (gd) => {
       JSON.stringify(exampleShortHeaders)
     );
 
+    await fs.writeFile(
+      path.join(databaseRootPath, 'themedStarters.json'),
+      JSON.stringify({ version: 1, themes: themedStarters })
+    );
+
+    await fs.writeFile(
+      path.join(databaseRootPath, 'themeSlots.json'),
+      JSON.stringify({
+        version: themeSlotsVocabulary.version,
+        slots: themeSlotsVocabulary.slots,
+        starters: themeSlotsByStarterSlug,
+      })
+    );
+
     await fs.writeFile(
       path.join(databaseRootPath, 'filters.json'),
       JSON.stringify(
diff --git a/scripts/lib/StarterThemer.js b/scripts/lib/StarterThemer.js
new file mode 100644
index 000000000..676b3f3b7
--- /dev/null
+++ b/scripts/lib/StarterThemer.js
@@ -0,0 +1,320 @@
+// @ts-check
+
+/** @typedef {import('./ThemeSlots.js').StarterThemeSlots} StarterThemeSlots */
+
+/**
+ * A theme, as published by the assets repository: for each slot, the file to
+ * use and, for a 3D model, the serialized content of the asset's object.
+ * @typedef {{
+ *   kind: 'model' | 'texture',
+ *   file: string,
+ *   objectContent?: any,
+ *   assetStoreId?: string,
+ *   origin?: {name: string, identifier: string},
+ * }} ThemeSlot
+ *
+ * @typedef {{
+ *   id: string,
+ *   name: string,
+ *   description: string,
+ *   slots: Object.<string, ThemeSlot>,
+ * }} Theme
+ *
+ * @typedef {{
+ *   appliedSlots: Array<string>,
+ *   missingSlots: Array<string>,
+ *   changedObjectsCount: number,
+ *   changedResourcesCount: number,
+ * }} ThemeApplicationResult
+ */
+
+/** @type {Object.<string, string>} */
+const cubeFaceProperties = {
+  front: 'frontFaceResourceName',
+  back: 'backFaceResourceName',
+  left: 'leftFaceResourceName',
+  right: 'rightFaceResourceName',
+  top: 'topFaceResourceName',
+  bottom: 'bottomFaceResourceName',
+};
+
+/**
+ * Every object of a serialized project, by the path a starter's
+ * theme-slots.json refers to it by.
+ * @param {any} projectObject
+ * @returns {Map<string, any>}
+ */
+const getObjectsByPath = (projectObject) => {
+  /** @type {Map<string, any>} */
+  const objectsByPath = new Map();
+  /** @param {string} prefix @param {Array<any> | undefined} objects */
+  const collect = (prefix, objects) =>
+    (objects || []).forEach((object) =>
+      objectsByPath.set(`${prefix}/${object.name}`, object)
+    );
+
+  collect('global', projectObject.objects);
+  (projectObject.layouts || []).forEach(
+    /** @param {any} layout */ (layout) =>
+      collect(`scene:${layout.name}`, layout.objects)
+  );
+  (projectObject.eventsFunctionsExtensions || []).forEach(
+    /** @param {any} extension */ (extension) =>
+      (extension.eventsBasedObjects || []).forEach(
+        /** @param {any} eventsBasedObject */ (eventsBasedObject) =>
+          collect(
+            `object:${extension.name}::${eventsBasedObject.name}`,
+            eventsBasedObject.objects
+          )
+      )
+  );
+  return objectsByPath;
+};
+
+/**
+ * @param {any} projectObject
+ * @returns {Map<string, any>}
+ */
+const getEffectsByPath = (projectObject) => {
+  /** @type {Map<string, any>} */
+  const effectsByPath = new Map();
+  (projectObject.layouts || []).forEach(
+    /** @param {any} layout */ (layout) =>
+      (layout.layers || []).forEach(
+        /** @param {any} layer */ (layer) =>
+          (layer.effects || []).forEach(
+            /** @param {any} effect */ (effect) =>
+              effectsByPath.set(
+                `effect:${layout.name}/${layer.name}/${effect.name}`,
+                effect
+              )
+          )
+      )
+  );
+  return effectsByPath;
+};
+
+/**
+ * Same merge as the asset swapper of the editor: the object keeps every
+ * animation name it had, played by the theme's animation of the same name (or
+ * its first one), and the theme's extra animations are appended.
+ * @param {Array<any>} objectAnimations
+ * @param {Array<any>} themeAnimations
+ * @returns {Array<any>}
+ */
+const mergeModel3DAnimations = (objectAnimations, themeAnimations) => {
+  if (!themeAnimations.length) return objectAnimations;
+
+  const animations = objectAnimations.map(
+    (objectAnimation) =>
+      themeAnimations.find(
+        (themeAnimation) => themeAnimation.name === objectAnimation.name
+      ) || { ...themeAnimations[0], name: objectAnimation.name }
+  );
+  themeAnimations.forEach((themeAnimation) => {
+    const isAlreadyAdded = objectAnimations.some(
+      (objectAnimation) => objectAnimation.name === themeAnimation.name
+    );
+    if (!isAlreadyAdded) animations.push(themeAnimation);
+  });
+  return animations;
+};
+
+/**
+ * Scale the theme model to the volume the placeholder occupied, so instances
+ * keep their footprint in the scene.
+ * @param {any} objectContent
+ * @param {any} themeContent
+ * @returns {number}
+ */
+const getSizeRatio = (objectContent, themeContent) => {
+  const objectVolume =
+    (objectContent.width || 0) *
+    (objectContent.height || 0) *
+    (objectContent.depth || 0);
+  const themeVolume =
+    (themeContent.width || 0) *
+    (themeContent.height || 0) *
+    (themeContent.depth || 0);
+  if (objectVolume <= 0 || themeVolume <= 0) return 1;
+  return Math.pow(objectVolume / themeVolume, 1 / 3);
+};
+
+/**
+ * Re-skin a starter with a theme by rewriting its serialized project, in place.
+ *
+ * The starter's theme-slots.json says which of its objects play which slot.
+ * Each such object gets the theme's asset for that slot: a 3D model takes the
+ * theme model's dimensions, rotation, material and animations while keeping
+ * its name, behaviors, variables, instances, origin and center; a cube face or
+ * a skybox face points at the theme image.
+ *
+ * Resources are repointed in place, keeping their names, so nothing else in
+ * the project has to be renamed.
+ *
+ * @param {any} projectObject
+ * @param {StarterThemeSlots} starterThemeSlots
+ * @param {Theme} theme
+ * @returns {ThemeApplicationResult}
+ */
+const applyThemeToStarter = (projectObject, starterThemeSlots, theme) => {
+  /** @type {Array<any>} */
+  const resources = (projectObject.resources || {}).resources || [];
+  /** @type {Map<string, any>} */
+  const resourceByName = new Map(
+    resources.map((resource) => [resource.name, resource])
+  );
+  const objectsByPath = getObjectsByPath(projectObject);
+  const effectsByPath = getEffectsByPath(projectObject);
+
+  /** @type {Set<string>} */
+  const appliedSlots = new Set();
+  /** @type {Set<string>} */
+  const missingSlots = new Set();
+  /** @type {Set<string>} */
+  const changedResourceNames = new Set();
+  /** @type {Map<string, string>} */
+  const slotIdByResourceName = new Map();
+  let changedObjectsCount = 0;
+
+  /**
+   * Point the resource of that name at the theme file and return the name to
+   * reference. Objects sharing a resource but playing different slots each get
+   * their own copy, so one does not overwrite the other.
+   * @param {string} resourceName
+   * @param {string} slotId
+   * @returns {string | null}
+   */
+  const repointResource = (resourceName, slotId) => {
+    const slot = theme.slots[slotId];
+    if (!slot) {
+      missingSlots.add(slotId);
+      return null;
+    }
+    const resource = resourceByName.get(resourceName);
+    if (!resource) return null;
+
+    const alreadyAppliedSlotId = slotIdByResourceName.get(resourceName);
+    let themedResource = resource;
+    if (alreadyAppliedSlotId && alreadyAppliedSlotId !== slotId) {
+      const copyName = `${resourceName} (${slotId})`;
+      themedResource = resourceByName.get(copyName) || {
+        ...resource,
+        name: copyName,
+      };
+      if (!resourceByName.has(copyName)) {
+        resources.push(themedResource);
+        resourceByName.set(copyName, themedResource);
+      }
+    }
+
+    themedResource.file = slot.file;
+    themedResource.origin = slot.origin || {
+      name: 'gdevelop-asset-store',
+      identifier: slot.file,
+    };
+    slotIdByResourceName.set(themedResource.name, slotId);
+    appliedSlots.add(slotId);
+    changedResourceNames.add(themedResource.name);
+    return themedResource.name;
+  };
+
+  Object.entries(starterThemeSlots.objects || {}).forEach(
+    ([objectPath, mapping]) => {
+      const object = objectsByPath.get(objectPath);
+      if (!object || !object.content) return;
+      const objectContent = object.content;
+
+      if (object.type === 'Scene3D::Model3DObject') {
+        if (typeof mapping !== 'string') return;
+        const slot = theme.slots[mapping];
+        if (!slot || slot.kind !== 'model' || !slot.objectContent) {
+          missingSlots.add(mapping);
+          return;
+        }
+        const modelResourceName = repointResource(
+          objectContent.modelResourceName,
+          mapping
+        );
+        if (!modelResourceName) return;
+
+        const themeContent = slot.objectContent;
+        const sizeRatio = getSizeRatio(objectContent, themeContent);
+        object.content = {
+          ...themeContent,
+          // Keep pointing at the project's own resource, now holding the theme model.
+          modelResourceName,
+          animations: mergeModel3DAnimations(
+            objectContent.animations || [],
+            themeContent.animations || []
+          ),
+          width: (themeContent.width || 0) * sizeRatio,
+          height: (themeContent.height || 0) * sizeRatio,
+          depth: (themeContent.depth || 0) * sizeRatio,
+          // The origin and center drive collisions and placement: keep the
+          // starter's, which its events and instances were built around.
+          originLocation: objectContent.originLocation,
+          centerLocation: objectContent.centerLocation,
+        };
+        if (slot.assetStoreId) object.assetStoreId = slot.assetStoreId;
+        changedObjectsCount++;
+        return;
+      }
+
+      if (object.type === 'Scene3D::Cube3DObject') {
+        let changed = false;
+        Object.keys(cubeFaceProperties).forEach((face) => {
+          const slotId = typeof mapping === 'string' ? mapping : mapping[face];
+          const faceProperty = cubeFaceProperties[face];
+          const resourceName = objectContent[faceProperty];
+          if (!slotId || !resourceName) return;
+          const themedResourceName = repointResource(resourceName, slotId);
+          if (!themedResourceName) return;
+          objectContent[faceProperty] = themedResourceName;
+          changed = true;
+        });
+        if (changed) changedObjectsCount++;
+      }
+    }
+  );
+
+  Object.entries(starterThemeSlots.effects || {}).forEach(
+    ([effectPath, parameters]) => {
+      const effect = effectsByPath.get(effectPath);
+      if (!effect || !effect.stringParameters) return;
+      Object.entries(parameters).forEach(([parameter, slotId]) => {
+        const resourceName = effect.stringParameters[parameter];
+        if (!resourceName) return;
+        const themedResourceName = repointResource(
+          String(resourceName),
+          slotId
+        );
+        if (themedResourceName) {
+          effect.stringParameters[parameter] = themedResourceName;
+        }
+      });
+    }
+  );
+
+  return {
+    appliedSlots: [...appliedSlots].sort(),
+    missingSlots: [...missingSlots].sort(),
+    changedObjectsCount,
+    changedResourcesCount: changedResourceNames.size,
+  };
+};
+
+/**
+ * The file a themed copy of a starter is written to: next to the starter, so
+ * the resources it refers to by a relative path still resolve.
+ * @param {string} slug
+ * @param {string} themeId
+ * @returns {string}
+ */
+const getThemedStarterFileName = (slug, themeId) =>
+  `${slug}.theme-${themeId}.json`;
+
+module.exports = {
+  applyThemeToStarter,
+  getThemedStarterFileName,
+};
diff --git a/scripts/lib/ThemeSlots.js b/scripts/lib/ThemeSlots.js
new file mode 100644
index 000000000..c424778a5
--- /dev/null
+++ b/scripts/lib/ThemeSlots.js
@@ -0,0 +1,236 @@
+// @ts-check
+const path = require('path');
+const fs = require('fs').promises;
+
+/**
+ * The vocabulary of slots a theme can fill, shared by every starter.
+ * @typedef {{
+ *   version: number,
+ *   slots: Array<{id: string, kind: 'model' | 'texture', label: string}>,
+ * }} ThemeSlotsVocabulary
+ */
+
+/**
+ * A starter's own mapping, next to its game file: which of its objects a theme
+ * re-skins, and as what. Object paths are `scene:<scene>/<object>`,
+ * `global/<object>` or `object:<Extension>::<CustomObject>/<child>`. A cube
+ * maps either to one texture slot for all six faces, or to one slot per face.
+ * @typedef {{
+ *   version: number,
+ *   objects: Object.<string, string | Object.<string, string>>,
+ *   effects?: Object.<string, Object.<string, string>>,
+ *   ignoredObjects?: Array<string>,
+ * }} StarterThemeSlots
+ */
+
+const vocabularyPath = path.join(__dirname, '../../theme-slots.json');
+
+/** @returns {Promise<ThemeSlotsVocabulary>} */
+const loadThemeSlotsVocabulary = async () =>
+  JSON.parse(await fs.readFile(vocabularyPath, 'utf8'));
+
+const cubeFaceProperties = [
+  'frontFaceResourceName',
+  'backFaceResourceName',
+  'leftFaceResourceName',
+  'rightFaceResourceName',
+  'topFaceResourceName',
+  'bottomFaceResourceName',
+];
+
+/**
+ * Every object of a project a theme could re-skin, with the path a mapping
+ * refers to it by, plus every skybox effect.
+ * @param {any} projectObject
+ * @returns {{objects: Array<{path: string, type: string}>, effects: Array<string>, is3D: boolean}}
+ */
+const getThemeableObjects = (projectObject) => {
+  /** @type {Array<{path: string, type: string}>} */
+  const objects = [];
+  /** @type {Array<string>} */
+  const effects = [];
+  const themeableTypes = ['Scene3D::Model3DObject', 'Scene3D::Cube3DObject'];
+
+  /** @param {string} prefix @param {Array<any>} list */
+  const collect = (prefix, list) =>
+    (list || []).forEach((object) => {
+      if (themeableTypes.includes(object.type)) {
+        objects.push({ path: `${prefix}/${object.name}`, type: object.type });
+      }
+    });
+
+  collect('global', projectObject.objects);
+  (projectObject.layouts || []).forEach(
+    /** @param {any} layout */ (layout) => {
+      collect(`scene:${layout.name}`, layout.objects);
+      (layout.layers || []).forEach(
+        /** @param {any} layer */ (layer) =>
+          (layer.effects || []).forEach(
+            /** @param {any} effect */ (effect) => {
+              if ((effect.effectType || '').includes('Skybox')) {
+                effects.push(
+                  `effect:${layout.name}/${layer.name}/${effect.name}`
+                );
+              }
+            }
+          )
+      );
+    }
+  );
+  (projectObject.eventsFunctionsExtensions || []).forEach(
+    /** @param {any} extension */ (extension) =>
+      (extension.eventsBasedObjects || []).forEach(
+        /** @param {any} eventsBasedObject */ (eventsBasedObject) =>
+          collect(
+            `object:${extension.name}::${eventsBasedObject.name}`,
+            eventsBasedObject.objects
+          )
+      )
+  );
+
+  return { objects, effects, is3D: objects.length > 0 || effects.length > 0 };
+};
+
+/**
+ * A 3D starter must say, for each object a theme could re-skin, which slot it
+ * plays or that it is deliberately left alone; and must not refer to objects
+ * or slots that do not exist. Enforced by the build so a starter cannot ship
+ * art the themes don't know how to replace.
+ * @param {ThemeSlotsVocabulary} vocabulary
+ * @param {string} slug
+ * @param {any} projectObject
+ * @param {StarterThemeSlots | null} starterThemeSlots
+ * @returns {Error[]}
+ */
+const checkStarterThemeSlots = (
+  vocabulary,
+  slug,
+  projectObject,
+  starterThemeSlots
+) => {
+  /** @type {Error[]} */
+  const errors = [];
+  const { objects, effects, is3D } = getThemeableObjects(projectObject);
+  if (!is3D) return errors;
+
+  if (!starterThemeSlots) {
+    errors.push(
+      new Error(
+        `Starter "${slug}" is a 3D starter but has no theme-slots.json: add one mapping its objects to theme slots (see the README).`
+      )
+    );
+    return errors;
+  }
+
+  const slotKindById = new Map(
+    vocabulary.slots.map((slot) => [slot.id, slot.kind])
+  );
+  const typeByPath = new Map(
+    objects.map((object) => [object.path, object.type])
+  );
+  const effectPaths = new Set(effects);
+  const mapped = starterThemeSlots.objects || {};
+  const ignored = new Set(starterThemeSlots.ignoredObjects || []);
+  const mappedEffects = starterThemeSlots.effects || {};
+
+  /** @param {string} where @param {string} slotId @param {'model'|'texture'} kind */
+  const checkSlot = (where, slotId, kind) => {
+    const declaredKind = slotKindById.get(slotId);
+    if (!declaredKind) {
+      errors.push(
+        new Error(
+          `Starter "${slug}": ${where} maps to "${slotId}", which is not a slot of theme-slots.json.`
+        )
+      );
+    } else if (declaredKind !== kind) {
+      errors.push(
+        new Error(
+          `Starter "${slug}": ${where} maps to "${slotId}", a ${declaredKind} slot, but needs a ${kind} slot.`
+        )
+      );
+    }
+  };
+
+  objects.forEach(({ path: objectPath, type }) => {
+    if (ignored.has(objectPath)) return;
+    const mapping = mapped[objectPath];
+    if (mapping === undefined) {
+      errors.push(
+        new Error(
+          `Starter "${slug}": the ${type} "${objectPath}" is neither mapped to a slot nor listed in ignoredObjects in theme-slots.json.`
+        )
+      );
+      return;
+    }
+    if (type === 'Scene3D::Model3DObject') {
+      if (typeof mapping !== 'string') {
+        errors.push(
+          new Error(
+            `Starter "${slug}": "${objectPath}" is a 3D model and must map to a single model slot.`
+          )
+        );
+        return;
+      }
+      checkSlot(`"${objectPath}"`, mapping, 'model');
+    } else if (typeof mapping === 'string') {
+      checkSlot(`"${objectPath}"`, mapping, 'texture');
+    } else {
+      Object.entries(mapping).forEach(([face, slotId]) => {
+        if (!cubeFaceProperties.some((property) => property.startsWith(face))) {
+          errors.push(
+            new Error(
+              `Starter "${slug}": "${objectPath}" maps the unknown cube face "${face}".`
+            )
+          );
+          return;
+        }
+        checkSlot(`"${objectPath}" face "${face}"`, slotId, 'texture');
+      });
+    }
+  });
+
+  Object.keys(mapped).forEach((objectPath) => {
+    if (!typeByPath.has(objectPath)) {
+      errors.push(
+        new Error(
+          `Starter "${slug}": theme-slots.json maps "${objectPath}", which is not a 3D object of the starter anymore.`
+        )
+      );
+    }
+  });
+  ignored.forEach((objectPath) => {
+    if (!typeByPath.has(objectPath)) {
+      errors.push(
+        new Error(
+          `Starter "${slug}": theme-slots.json ignores "${objectPath}", which is not a 3D object of the starter anymore.`
+        )
+      );
+    }
+  });
+  Object.entries(mappedEffects).forEach(([effectPath, faces]) => {
+    if (!effectPaths.has(effectPath)) {
+      errors.push(
+        new Error(
+          `Starter "${slug}": theme-slots.json maps the effect "${effectPath}", which is not a skybox of the starter anymore.`
+        )
+      );
+      return;
+    }
+    Object.entries(faces).forEach(([parameter, slotId]) =>
+      checkSlot(
+        `skybox "${effectPath}" parameter "${parameter}"`,
+        slotId,
+        'texture'
+      )
+    );
+  });
+
+  return errors;
+};
+
+module.exports = {
+  loadThemeSlotsVocabulary,
+  getThemeableObjects,
+  checkStarterThemeSlots,
+  cubeFaceProperties,
+};
diff --git a/scripts/lib/__tests__/StarterThemer.spec.js b/scripts/lib/__tests__/StarterThemer.spec.js
new file mode 100644
index 000000000..e20b6901c
--- /dev/null
+++ b/scripts/lib/__tests__/StarterThemer.spec.js
@@ -0,0 +1,412 @@
+// @ts-check
+const {
+  applyThemeToStarter,
+  getThemedStarterFileName,
+} = require('../StarterThemer');
+
+/** @typedef {import('../ThemeSlots.js').StarterThemeSlots} StarterThemeSlots */
+/** @typedef {import('../StarterThemer.js').Theme} Theme */
+
+/** @returns {StarterThemeSlots} */
+const makeStarterThemeSlots = () => ({
+  version: 1,
+  objects: {
+    'scene:Game Scene/Player': 'character.player',
+    'scene:Game Scene/Enemy': 'character.enemy',
+    'scene:Game Scene/Ground': 'env.ground',
+    'scene:Game Scene/Crate': { top: 'env.ground', front: 'env.wall' },
+    'object:TankConfiguration::CombinedTank/TankBase': 'character.player',
+  },
+  effects: {
+    'effect:Game Scene//SkyBox': { frontFaceResourceName: 'sky.day.front' },
+  },
+  ignoredObjects: ['scene:Game Scene/Camera'],
+});
+
+/** @returns {Theme} */
+const makeStarterTheme = () => ({
+  id: 'pirate',
+  name: 'Pirate islands',
+  description: 'Tropical islands.',
+  slots: {
+    'character.player': {
+      kind: 'model',
+      file: 'https://asset-resources.gdevelop.io/public-resources/Henry.glb',
+      assetStoreId: 'abc123',
+      objectContent: {
+        modelResourceName: 'Henry.glb',
+        width: 200,
+        height: 200,
+        depth: 200,
+        rotationX: 90,
+        rotationY: 0,
+        rotationZ: 90,
+        materialType: 'StandardWithoutMetalness',
+        originLocation: 'ModelOrigin',
+        centerLocation: 'CenteredOnZ',
+        animations: [
+          { name: 'Idle', source: 'Henry_Idle', loop: true },
+          { name: 'Run', source: 'Henry_Run', loop: true },
+        ],
+      },
+    },
+    'env.ground': {
+      kind: 'texture',
+      file: 'https://asset-resources.gdevelop.io/public-resources/Sand.png',
+    },
+    'env.wall': {
+      kind: 'texture',
+      file: 'https://asset-resources.gdevelop.io/public-resources/Planks.png',
+    },
+    'sky.day.front': {
+      kind: 'texture',
+      file: 'https://asset-resources.gdevelop.io/public-resources/Tropical.png',
+      origin: { name: 'gdevelop-asset-store', identifier: 'tropical-front' },
+    },
+  },
+});
+
+/** @param {string} modelResourceName */
+const makeModelContent = (modelResourceName) => ({
+  modelResourceName,
+  width: 100,
+  height: 100,
+  depth: 100,
+  originLocation: 'ModelOrigin',
+  centerLocation: 'ModelOrigin',
+  animations: [],
+});
+
+/** @returns {any} */
+const makeProjectContent = () => ({
+  resources: {
+    resources: [
+      {
+        name: 'unit_orange.glb',
+        file: 'assets/unit_orange.glb',
+        kind: 'model3D',
+      },
+      { name: 'unit_red.glb', file: 'assets/unit_red.glb', kind: 'model3D' },
+      { name: 'Ground.png', file: 'assets/Ground.png', kind: 'image' },
+      { name: 'Wall.png', file: 'assets/Wall.png', kind: 'image' },
+      { name: 'Sky_Front.png', file: 'assets/Sky_Front.png', kind: 'image' },
+      { name: 'Camera.png', file: 'assets/Camera.png', kind: 'image' },
+    ],
+  },
+  layouts: [
+    {
+      name: 'Game Scene',
+      layers: [
+        {
+          name: '',
+          effects: [
+            {
+              name: 'SkyBox',
+              effectType: 'Scene3D::Skybox',
+              stringParameters: { frontFaceResourceName: 'Sky_Front.png' },
+            },
+          ],
+        },
+      ],
+      objects: [
+        {
+          name: 'Player',
+          type: 'Scene3D::Model3DObject',
+          behaviors: [
+            { name: 'Physics3D', type: 'Physics3D::Physics3DBehavior' },
+          ],
+          variables: [{ name: 'Health', value: 3 }],
+          content: makeModelContent('unit_orange.glb'),
+        },
+        {
+          name: 'Enemy',
+          type: 'Scene3D::Model3DObject',
+          content: makeModelContent('unit_red.glb'),
+        },
+        {
+          name: 'Ground',
+          type: 'Scene3D::Cube3DObject',
+          content: {
+            frontFaceResourceName: 'Ground.png',
+            topFaceResourceName: 'Ground.png',
+          },
+        },
+        {
+          name: 'Crate',
+          type: 'Scene3D::Cube3DObject',
+          content: {
+            frontFaceResourceName: 'Wall.png',
+            topFaceResourceName: 'Ground.png',
+          },
+        },
+        {
+          name: 'Camera',
+          type: 'Scene3D::Cube3DObject',
+          content: { frontFaceResourceName: 'Camera.png' },
+        },
+      ],
+    },
+  ],
+  eventsFunctionsExtensions: [
+    {
+      name: 'TankConfiguration',
+      eventsBasedObjects: [
+        {
+          name: 'CombinedTank',
+          objects: [
+            {
+              name: 'TankBase',
+              type: 'Scene3D::Model3DObject',
+              content: makeModelContent('unit_orange.glb'),
+            },
+          ],
+        },
+      ],
+    },
+  ],
+});
+
+/** @param {any} projectContent */
+const applyPirateTheme = (projectContent) =>
+  applyThemeToStarter(
+    projectContent,
+    makeStarterThemeSlots(),
+    makeStarterTheme()
+  );
+
+describe('applyThemeToStarter', () => {
+  it('repoints the resources of mapped objects at the theme files', () => {
+    const projectContent = makeProjectContent();
+    applyPirateTheme(projectContent);
+
+    const resources = projectContent.resources.resources;
+    expect(resources[0].file).toBe(
+      'https://asset-resources.gdevelop.io/public-resources/Henry.glb'
+    );
+    expect(resources[2].file).toBe(
+      'https://asset-resources.gdevelop.io/public-resources/Sand.png'
+    );
+    expect(resources[4].file).toBe(
+      'https://asset-resources.gdevelop.io/public-resources/Tropical.png'
+    );
+    expect(resources[4].origin).toEqual({
+      name: 'gdevelop-asset-store',
+      identifier: 'tropical-front',
+    });
+  });
+
+  it('does not touch the resources of ignored or unmapped objects', () => {
+    const projectContent = makeProjectContent();
+    applyPirateTheme(projectContent);
+
+    expect(projectContent.resources.resources[5].file).toBe(
+      'assets/Camera.png'
+    );
+  });
+
+  it('keeps resource names so every reference follows', () => {
+    const projectContent = makeProjectContent();
+    applyPirateTheme(projectContent);
+
+    expect(projectContent.resources.resources[2].name).toBe('Ground.png');
+    expect(
+      projectContent.layouts[0].objects[2].content.frontFaceResourceName
+    ).toBe('Ground.png');
+  });
+
+  it('takes the theme model content while keeping the object identity', () => {
+    const projectContent = makeProjectContent();
+    applyPirateTheme(projectContent);
+
+    const player = projectContent.layouts[0].objects[0];
+    expect(player.name).toBe('Player');
+    expect(player.behaviors).toEqual([
+      { name: 'Physics3D', type: 'Physics3D::Physics3DBehavior' },
+    ]);
+    expect(player.variables).toEqual([{ name: 'Health', value: 3 }]);
+    expect(player.assetStoreId).toBe('abc123');
+    expect(player.content.materialType).toBe('StandardWithoutMetalness');
+    expect(player.content.modelResourceName).toBe('unit_orange.glb');
+    expect(player.content.originLocation).toBe('ModelOrigin');
+    expect(player.content.centerLocation).toBe('ModelOrigin');
+  });
+
+  it('scales the theme model to the volume the placeholder occupied', () => {
+    const projectContent = makeProjectContent();
+    applyPirateTheme(projectContent);
+
+    const player = projectContent.layouts[0].objects[0];
+    expect(player.content.width).toBeCloseTo(100);
+    expect(player.content.height).toBeCloseTo(100);
+    expect(player.content.depth).toBeCloseTo(100);
+  });
+
+  it('gives the object the theme animations when it had none', () => {
+    const projectContent = makeProjectContent();
+    applyPirateTheme(projectContent);
+
+    expect(projectContent.layouts[0].objects[0].content.animations).toEqual([
+      { name: 'Idle', source: 'Henry_Idle', loop: true },
+      { name: 'Run', source: 'Henry_Run', loop: true },
+    ]);
+  });
+
+  it('keeps the animation names the object already had', () => {
+    const projectContent = makeProjectContent();
+    projectContent.layouts[0].objects[0].content.animations = [
+      { name: 'Run', source: 'Placeholder_Run', loop: true },
+      { name: 'Jump', source: 'Placeholder_Jump', loop: false },
+    ];
+    applyPirateTheme(projectContent);
+
+    const animations = projectContent.layouts[0].objects[0].content.animations;
+    expect(
+      animations.map(
+        /** @param {any} animation */ (animation) => animation.name
+      )
+    ).toEqual(['Run', 'Jump', 'Idle']);
+    expect(animations[0].source).toBe('Henry_Run');
+    // An animation the theme does not have falls back to its first one.
+    expect(animations[1].source).toBe('Henry_Idle');
+  });
+
+  it('lets a cube map each face to a different slot', () => {
+    const projectContent = makeProjectContent();
+    const result = applyPirateTheme(projectContent);
+
+    expect(projectContent.resources.resources[3].file).toBe(
+      'https://asset-resources.gdevelop.io/public-resources/Planks.png'
+    );
+    expect(result.appliedSlots).toContain('env.wall');
+  });
+
+  it('themes the 3D objects held by events-based objects', () => {
+    const projectContent = makeProjectContent();
+    const result = applyPirateTheme(projectContent);
+
+    const tankBase =
+      projectContent.eventsFunctionsExtensions[0].eventsBasedObjects[0]
+        .objects[0];
+    expect(tankBase.content.materialType).toBe('StandardWithoutMetalness');
+    // Player, Ground, Crate and TankBase: the theme has no enemy.
+    expect(result.changedObjectsCount).toBe(4);
+  });
+
+  it('reports the slots the theme does not fill and leaves them untouched', () => {
+    const projectContent = makeProjectContent();
+    const result = applyPirateTheme(projectContent);
+
+    expect(result.missingSlots).toEqual(['character.enemy']);
+    expect(projectContent.resources.resources[1].file).toBe(
+      'assets/unit_red.glb'
+    );
+  });
+
+  it('gives objects sharing a resource their own copy when they play different slots', () => {
+    const projectContent = makeProjectContent();
+    // The enemy uses the same model file as the player.
+    projectContent.layouts[0].objects[1].content.modelResourceName =
+      'unit_orange.glb';
+    const starterTheme = makeStarterTheme();
+    starterTheme.slots['character.enemy'] = {
+      kind: 'model',
+      file: 'https://asset-resources.gdevelop.io/public-resources/Skeleton.glb',
+      objectContent: {
+        modelResourceName: 'Skeleton.glb',
+        width: 100,
+        height: 100,
+        depth: 100,
+        animations: [],
+      },
+    };
+
+    applyThemeToStarter(projectContent, makeStarterThemeSlots(), starterTheme);
+
+    const [player, enemy] = projectContent.layouts[0].objects;
+    /** @type {Object.<string, any>} */
+    const resourceByName = {};
+    projectContent.resources.resources.forEach(
+      /** @param {any} resource */ (resource) => {
+        resourceByName[resource.name] = resource;
+      }
+    );
+    expect(player.content.modelResourceName).toBe('unit_orange.glb');
+    expect(resourceByName['unit_orange.glb'].file).toBe(
+      'https://asset-resources.gdevelop.io/public-resources/Henry.glb'
+    );
+    expect(enemy.content.modelResourceName).toBe(
+      'unit_orange.glb (character.enemy)'
+    );
+    expect(resourceByName['unit_orange.glb (character.enemy)'].file).toBe(
+      'https://asset-resources.gdevelop.io/public-resources/Skeleton.glb'
+    );
+  });
+
+  it('skips a mapped object the project does not have', () => {
+    const projectContent = makeProjectContent();
+    const starterThemeSlots = makeStarterThemeSlots();
+    starterThemeSlots.objects['scene:Game Scene/Ghost'] = 'character.player';
+
+    const result = applyThemeToStarter(
+      projectContent,
+      starterThemeSlots,
+      makeStarterTheme()
+    );
+
+    expect(result.changedObjectsCount).toBe(4);
+  });
+
+  it('can be applied again to change the theme of an already themed project', () => {
+    const projectContent = makeProjectContent();
+    applyPirateTheme(projectContent);
+
+    const secondTheme = makeStarterTheme();
+    secondTheme.id = 'medieval';
+    secondTheme.slots['character.player'] = {
+      kind: 'model',
+      file: 'https://asset-resources.gdevelop.io/public-resources/Knight.glb',
+      objectContent: {
+        modelResourceName: 'Knight.glb',
+        width: 150,
+        height: 150,
+        depth: 150,
+        animations: [{ name: 'Idle', source: 'Knight_Idle', loop: true }],
+      },
+    };
+    const result = applyThemeToStarter(
+      projectContent,
+      makeStarterThemeSlots(),
+      secondTheme
+    );
+
+    expect(projectContent.resources.resources[0].file).toBe(
+      'https://asset-resources.gdevelop.io/public-resources/Knight.glb'
+    );
+    expect(
+      projectContent.layouts[0].objects[0].content.animations[0].source
+    ).toBe('Knight_Idle');
+  });
+
+  it('changes nothing when the theme fills no slot of the starter', () => {
+    const projectContent = makeProjectContent();
+    const result = applyThemeToStarter(
+      projectContent,
+      makeStarterThemeSlots(),
+      { id: 'empty', name: 'Empty', description: '', slots: {} }
+    );
+
+    expect(result.changedObjectsCount).toBe(0);
+    expect(result.changedResourcesCount).toBe(0);
+    expect(projectContent.resources.resources[0].file).toBe(
+      'assets/unit_orange.glb'
+    );
+  });
+});
+
+describe('getThemedStarterFileName', () => {
+  it('names the themed copy after the starter, to sit next to it', () => {
+    expect(getThemedStarterFileName('starting-3d-tank', 'pirate')).toBe(
+      'starting-3d-tank.theme-pirate.json'
+    );
+  });
+});
diff --git a/scripts/lib/__tests__/ThemeSlots.spec.js b/scripts/lib/__tests__/ThemeSlots.spec.js
new file mode 100644
index 000000000..aa87165f3
--- /dev/null
+++ b/scripts/lib/__tests__/ThemeSlots.spec.js
@@ -0,0 +1,211 @@
+// @ts-check
+const {
+  getThemeableObjects,
+  checkStarterThemeSlots,
+} = require('../ThemeSlots');
+
+/** @typedef {import('../ThemeSlots.js').ThemeSlotsVocabulary} ThemeSlotsVocabulary */
+/** @typedef {import('../ThemeSlots.js').StarterThemeSlots} StarterThemeSlots */
+
+/** @type {ThemeSlotsVocabulary} */
+const vocabulary = {
+  version: 1,
+  slots: [
+    { id: 'character.player', kind: 'model', label: 'Player character' },
+    { id: 'env.ground', kind: 'texture', label: 'Ground' },
+    { id: 'env.wall', kind: 'texture', label: 'Wall' },
+    { id: 'sky.day.front', kind: 'texture', label: 'Day skybox, front' },
+  ],
+};
+
+const createFakeProjectObject = () => ({
+  layouts: [
+    {
+      name: 'Game Scene',
+      layers: [
+        {
+          name: '',
+          effects: [{ name: 'SkyBox', effectType: 'Scene3D::Skybox' }],
+        },
+      ],
+      objects: [
+        { name: 'Player', type: 'Scene3D::Model3DObject' },
+        { name: 'Ground', type: 'Scene3D::Cube3DObject' },
+        { name: 'Camera', type: 'Scene3D::Cube3DObject' },
+        { name: 'Joystick', type: 'Sprite' },
+      ],
+    },
+  ],
+  eventsFunctionsExtensions: [
+    {
+      name: 'TankConfiguration',
+      eventsBasedObjects: [
+        {
+          name: 'CombinedTank',
+          objects: [{ name: 'TankBase', type: 'Scene3D::Model3DObject' }],
+        },
+      ],
+    },
+  ],
+});
+
+/** @returns {StarterThemeSlots} */
+const createFakeStarterThemeSlots = () => ({
+  version: 1,
+  objects: {
+    'scene:Game Scene/Player': 'character.player',
+    'scene:Game Scene/Ground': 'env.ground',
+    'object:TankConfiguration::CombinedTank/TankBase': 'character.player',
+  },
+  effects: {
+    'effect:Game Scene//SkyBox': { frontFaceResourceName: 'sky.day.front' },
+  },
+  ignoredObjects: ['scene:Game Scene/Camera'],
+});
+
+describe('getThemeableObjects', () => {
+  it('lists the 3D objects of scenes and custom objects, and the skyboxes', () => {
+    const { objects, effects, is3D } = getThemeableObjects(
+      createFakeProjectObject()
+    );
+
+    expect(is3D).toBe(true);
+    expect(objects.map((object) => object.path)).toEqual([
+      'scene:Game Scene/Player',
+      'scene:Game Scene/Ground',
+      'scene:Game Scene/Camera',
+      'object:TankConfiguration::CombinedTank/TankBase',
+    ]);
+    expect(effects).toEqual(['effect:Game Scene//SkyBox']);
+  });
+
+  it('does not consider a project without 3D objects as 3D', () => {
+    expect(
+      getThemeableObjects({
+        layouts: [{ name: 'Scene', objects: [{ name: 'A', type: 'Sprite' }] }],
+      }).is3D
+    ).toBe(false);
+  });
+});
+
+describe('checkStarterThemeSlots', () => {
+  it('accepts a starter whose 3D objects are all mapped or ignored', () => {
+    expect(
+      checkStarterThemeSlots(
+        vocabulary,
+        'starting-3d-test',
+        createFakeProjectObject(),
+        createFakeStarterThemeSlots()
+      )
+    ).toEqual([]);
+  });
+
+  it('asks for a theme-slots.json when a 3D starter has none', () => {
+    const errors = checkStarterThemeSlots(
+      vocabulary,
+      'starting-3d-test',
+      createFakeProjectObject(),
+      null
+    );
+
+    expect(errors).toHaveLength(1);
+    expect(errors[0].message).toContain('has no theme-slots.json');
+  });
+
+  it('asks nothing of a starter without 3D objects', () => {
+    expect(
+      checkStarterThemeSlots(
+        vocabulary,
+        'starting-2d-test',
+        {
+          layouts: [
+            { name: 'Scene', objects: [{ name: 'A', type: 'Sprite' }] },
+          ],
+        },
+        null
+      )
+    ).toEqual([]);
+  });
+
+  it('reports a 3D object that is neither mapped nor ignored', () => {
+    const starterThemeSlots = createFakeStarterThemeSlots();
+    delete starterThemeSlots.objects['scene:Game Scene/Ground'];
+
+    const errors = checkStarterThemeSlots(
+      vocabulary,
+      'starting-3d-test',
+      createFakeProjectObject(),
+      starterThemeSlots
+    );
+
+    expect(errors).toHaveLength(1);
+    expect(errors[0].message).toContain('"scene:Game Scene/Ground"');
+  });
+
+  it('reports a mapping to an object the starter does not have anymore', () => {
+    const starterThemeSlots = createFakeStarterThemeSlots();
+    starterThemeSlots.objects['scene:Game Scene/Ghost'] = 'character.player';
+
+    const errors = checkStarterThemeSlots(
+      vocabulary,
+      'starting-3d-test',
+      createFakeProjectObject(),
+      starterThemeSlots
+    );
+
+    expect(errors).toHaveLength(1);
+    expect(errors[0].message).toContain('"scene:Game Scene/Ghost"');
+  });
+
+  it('reports a slot that does not exist, or of the wrong kind', () => {
+    const starterThemeSlots = createFakeStarterThemeSlots();
+    starterThemeSlots.objects['scene:Game Scene/Player'] = 'character.hero';
+    starterThemeSlots.objects['scene:Game Scene/Ground'] = 'character.player';
+
+    const messages = checkStarterThemeSlots(
+      vocabulary,
+      'starting-3d-test',
+      createFakeProjectObject(),
+      starterThemeSlots
+    ).map((error) => error.message);
+
+    expect(messages).toEqual([
+      expect.stringContaining('"character.hero", which is not a slot'),
+      expect.stringContaining('a model slot, but needs a texture slot'),
+    ]);
+  });
+
+  it('lets a cube map each face to its own slot', () => {
+    const starterThemeSlots = createFakeStarterThemeSlots();
+    starterThemeSlots.objects['scene:Game Scene/Ground'] = {
+      top: 'env.ground',
+      front: 'env.wall',
+    };
+
+    expect(
+      checkStarterThemeSlots(
+        vocabulary,
+        'starting-3d-test',
+        createFakeProjectObject(),
+        starterThemeSlots
+      )
+    ).toEqual([]);
+  });
+
+  it('reports a skybox mapping to an effect the starter does not have', () => {
+    const starterThemeSlots = createFakeStarterThemeSlots();
+    starterThemeSlots.effects = {
+      'effect:Game Scene//Gone': { frontFaceResourceName: 'sky.day.front' },
+    };
+
+    const errors = checkStarterThemeSlots(
+      vocabulary,
+      'starting-3d-test',
+      createFakeProjectObject(),
+      starterThemeSlots
+    );
+
+    expect(errors).toHaveLength(1);
+    expect(errors[0].message).toContain('"effect:Game Scene//Gone"');
+  });
+});
diff --git a/theme-slots.json b/theme-slots.json
new file mode 100644
index 000000000..cba69eb29
--- /dev/null
+++ b/theme-slots.json
@@ -0,0 +1,235 @@
+{
+  "version": 1,
+  "slots": [
+    {
+      "id": "character.player",
+      "kind": "model",
+      "label": "Player character"
+    },
+    {
+      "id": "character.enemy",
+      "kind": "model",
+      "label": "Enemy character"
+    },
+    {
+      "id": "character.npc",
+      "kind": "model",
+      "label": "NPC or ally character"
+    },
+    {
+      "id": "character.monster",
+      "kind": "model",
+      "label": "Monster or creature"
+    },
+    {
+      "id": "vehicle.car",
+      "kind": "model",
+      "label": "Drivable ground vehicle"
+    },
+    {
+      "id": "vehicle.aircraft.player",
+      "kind": "model",
+      "label": "Player aircraft"
+    },
+    {
+      "id": "vehicle.aircraft.enemy",
+      "kind": "model",
+      "label": "Enemy aircraft"
+    },
+    {
+      "id": "vehicle.tank.base",
+      "kind": "model",
+      "label": "Turret vehicle base"
+    },
+    {
+      "id": "vehicle.tank.turret",
+      "kind": "model",
+      "label": "Turret vehicle turret"
+    },
+    {
+      "id": "vehicle.tank.cannon",
+      "kind": "model",
+      "label": "Turret vehicle cannon"
+    },
+    {
+      "id": "weapon.handheld",
+      "kind": "model",
+      "label": "First person weapon"
+    },
+    {
+      "id": "projectile.player",
+      "kind": "model",
+      "label": "Player projectile"
+    },
+    {
+      "id": "projectile.enemy",
+      "kind": "model",
+      "label": "Enemy projectile"
+    },
+    {
+      "id": "pickup.coin",
+      "kind": "model",
+      "label": "Coin or currency pickup"
+    },
+    {
+      "id": "hazard.spike",
+      "kind": "model",
+      "label": "Hazard or trap"
+    },
+    {
+      "id": "nature.tree",
+      "kind": "model",
+      "label": "Tree"
+    },
+    {
+      "id": "nature.grass",
+      "kind": "model",
+      "label": "Grass or bush"
+    },
+    {
+      "id": "nature.rock.big",
+      "kind": "model",
+      "label": "Large rock"
+    },
+    {
+      "id": "nature.rock.small",
+      "kind": "model",
+      "label": "Small rock"
+    },
+    {
+      "id": "resource.log",
+      "kind": "model",
+      "label": "Log or raw resource"
+    },
+    {
+      "id": "crop.a",
+      "kind": "model",
+      "label": "First crop or food item"
+    },
+    {
+      "id": "crop.b",
+      "kind": "model",
+      "label": "Second crop or food item"
+    },
+    {
+      "id": "prop.container",
+      "kind": "model",
+      "label": "Bag, crate or chest"
+    },
+    {
+      "id": "building.tower",
+      "kind": "model",
+      "label": "Tower or placeable building"
+    },
+    {
+      "id": "prop.obstacle",
+      "kind": "model",
+      "label": "Road or track obstacle"
+    },
+    {
+      "id": "env.ground",
+      "kind": "texture",
+      "label": "Ground"
+    },
+    {
+      "id": "env.ground.tiled",
+      "kind": "texture",
+      "label": "Ground with a tile grid"
+    },
+    {
+      "id": "env.wall",
+      "kind": "texture",
+      "label": "Wall and building faces"
+    },
+    {
+      "id": "env.road",
+      "kind": "texture",
+      "label": "Road or track surface"
+    },
+    {
+      "id": "env.water",
+      "kind": "texture",
+      "label": "Water or lava"
+    },
+    {
+      "id": "env.tiles",
+      "kind": "texture",
+      "label": "Placement grid overlay"
+    },
+    {
+      "id": "env.crate",
+      "kind": "texture",
+      "label": "Pushable box faces"
+    },
+    {
+      "id": "env.finish",
+      "kind": "texture",
+      "label": "Finish line"
+    },
+    {
+      "id": "env.dirt",
+      "kind": "texture",
+      "label": "Farm plot or diggable ground"
+    },
+    {
+      "id": "sky.day.front",
+      "kind": "texture",
+      "label": "Day skybox, front face"
+    },
+    {
+      "id": "sky.day.back",
+      "kind": "texture",
+      "label": "Day skybox, back face"
+    },
+    {
+      "id": "sky.day.left",
+      "kind": "texture",
+      "label": "Day skybox, left face"
+    },
+    {
+      "id": "sky.day.right",
+      "kind": "texture",
+      "label": "Day skybox, right face"
+    },
+    {
+      "id": "sky.day.top",
+      "kind": "texture",
+      "label": "Day skybox, top face"
+    },
+    {
+      "id": "sky.day.bottom",
+      "kind": "texture",
+      "label": "Day skybox, bottom face"
+    },
+    {
+      "id": "sky.dark.front",
+      "kind": "texture",
+      "label": "Dark skybox, front face"
+    },
+    {
+      "id": "sky.dark.back",
+      "kind": "texture",
+      "label": "Dark skybox, back face"
+    },
+    {
+      "id": "sky.dark.left",
+      "kind": "texture",
+      "label": "Dark skybox, left face"
+    },
+    {
+      "id": "sky.dark.right",
+      "kind": "texture",
+      "label": "Dark skybox, right face"
+    },
+    {
+      "id": "sky.dark.top",
+      "kind": "texture",
+      "label": "Dark skybox, top face"
+    },
+    {
+      "id": "sky.dark.bottom",
+      "kind": "texture",
+      "label": "Dark skybox, bottom face"
+    }
+  ]
+}
\ No newline at end of file