chore: Change flashbar styles for one-theme

[at-susie]

Description

This PR updates flashbar styles for the one-theme. Originally we merged this PR for the flashbar style changes, but there were issue detected in a pipeline so the #4671 was reverted in the #4683. This PR brings the flashbar style changes with the fix for the issues.

Related links, issue #, if available: n/a

How has this been tested?

Review checklist

The following items are to be evaluated by the author(s) and the reviewer(s).

Correctness

• Changes include appropriate documentation updates.
• Changes are backward-compatible if not indicated, see CONTRIBUTING.md.
• Changes do not include unsupported browser features, see CONTRIBUTING.md.
• Changes were manually tested for accessibility, see accessibility guidelines.

Security

• If the code handles URLs: all URLs are validated through the checkSafeUrl function.

Testing

• Changes are covered with new/existing unit tests?
• Changes are covered with new/existing integration tests?

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[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 97.57%. Comparing base (7f6f93b) to head (d217f83).

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #4686   +/-   ##
=======================================
  Coverage   97.57%   97.57%           
=======================================
  Files         948      948           
  Lines       30495    30502    +7     
  Branches    11154    11159    +5     
=======================================
+ Hits        29755    29762    +7     
  Misses        733      733           
  Partials        7        7           

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[at-susie]

@mxschll
In the new one-theme Flashbar design, the status icon gets its own color (green for success, red for error, etc.) that's different from the neutral body text. That per-status icon color is applied in src/flashbar/styles.scss.

The problem is that for the Core, builders can override colors at runtime through the Flashbar style API. But a color set on the root flash container can't reach the icon, because that class-based SCSS rule on .flash-icon beats anything inherited from the parent. So the only way to let a Core builders customize the icon color is to write it inline, directly on the icon element — and that's exactly what getFlashIconStyles does.

Nothing will be changed for the existing builders' flashbar who have customized flahsbar using the style API. So this change is safe.

[mxschll]

Why does the comment say icon color? Isnt this for the whole text?