Darkmode compatibility

[mwmwmw]

• Bugfix (non-breaking change which fixes an issue)
• Feature (non-breaking change which adds functionality)
• Code style update
• Refactor (refactoring or adding test which isn't a fix or add a feature)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• Build-related changes
• Other, please describe:

Does this PR introduce a breaking change? (check one)

• Yes
• No

Did you test your solution?

• I lightly tested it in one browser
• I deeply tested it in several browsers
• I wrote tests around it (unit tests, integration tests, E2E tests)

Problem Description

Equations cannot be read in dark mode due to dark text on dark background.

Solution Description

Include a background colour when querying LaTeX, so the equation is visible in dark mode.

Side Effects, Risks, Impact

I wasn't able to find a solution to the zero-margin image, so legibility is compromised.

The element equations are broken in master and not a result of this fix.