[SybelBlue]

Summary

This issue is affecting PrairieLearn downstream! We'd appreciate a swift resolution so we can revert our workaround.

This fix:

• makes theme="light" override prefers-color-scheme: dark for mathfield, shared UI, and menu colors
• preserves theme="dark" behavior on light systems
• keeps the theme selectors at the original specificity so forced-colors overrides remain authoritative
• adds focused cross-browser Playwright coverage for system, explicit theme, dynamic removal, and forced-colors behavior

Fixes #2999.
Supersedes #3000.

Prior art

• reteps' Honor theme="light" over prefers-color-scheme: dark #3000 established the :not([theme="light"]) direction and the initial theme test matrix
• Formula Editor: pl-symbolic-input Selection BG Color Goes Black PrairieLearn/PrairieLearn#14766 documents the downstream user-visible failure
• Keep the formula editor highlight legible in OS dark mode adam-s/PrairieLearn#52 independently reproduced and measured the downstream highlight workaround

This replacement is based on current master, omits the unrelated virtual-keyboard cleanup from #3000, and uses :where() to avoid raising selector specificity above the later forced-colors rules.

Testing

• Focused theme Playwright suite: 6/6 passing in Chromium, Firefox, and WebKit.
• Unit and declaration suites: passing (348 unit tests and 256 snapshots).
• Full Playwright suite: 271 passing, 4 skipped, with one pre-existing WebKit context-menu failure (mouse.spec.ts, expected x+y=30, received an empty value) that reproduces in isolation and is unrelated to these CSS changes.
• npm run lint reports the existing no-restricted-globals error for speechSynthesis in src/public/math-static-elements.ts; changed files are formatted.

[SybelBlue]

This was closed accidentally by our downstream workaround PR closing.