Respect explicit light theme

[nvi9]

Summary

When viewer-css-theme="LIGHT" is set, Paper & Ink’s dark media rules still apply whenever the system preferred dark. Wrapped both dark sections in a shared CSS guard so explicit LIGHT stays light.

viewer.css already uses CSS nesting, so this change should have limited additional impact on browser compatibility.

However, in older browsers without nesting support, AUTOMATIC or an unset viewer-css-theme previously followed the system theme and will now remain light. Explicit LIGHT and DARK modes still work. If this change is not acceptable, we can use flat selectors instead to preserve automatic theme switching in older browsers, though that would require changing a bit more CSS.

Validation

> pnpm test -- --run                                                                                                                             
$ pnpm sync:pdfjs && vitest -- --run
$ node scripts/sync-pdfjs-assets.mjs
[sync-pdfjs] Synced PDF.js assets for v6.4.299 (legacy build)

 DEV  v5.0.4 ./pdfjs-viewer

 ✓  firefox  tests/basic.test.ts (15 tests) 5155ms
   ✓ Basic tests (15)
     ✓ should render the PDF file 1519ms
     ✓ should not render PDF with wrong url 274ms
     ✓ should open the external file with base64 source 289ms
     ✓ should load the dark theme 256ms
     ✓ should keep LIGHT fixed while automatic mode follows the system theme 260ms
     ✓ should hide the download button 218ms
     ✓ should add a title attribute to the iframe 234ms
     ✓ should apply hash-related attributes at runtime 235ms
     ✓ should apply worker-src option at runtime 249ms
     ✓ should include external worker origin in CSP script and worker directives 252ms
     ✓ should load runtime files from assets-base 473ms
     ✓ should prefer worker-src over assets-base and fall back to it on remove 150ms
     ✓ should use built-in worker by default 261ms
     ✓ should apply extended option attributes at runtime and restore defaults on remove 249ms
     ✓ should open a new document when src changes at runtime 229ms

 Test Files  1 passed (1)
      Tests  15 passed (15)
   Start at  11:55:03
   Duration  12.51s (tests 54%, worker 46%)

 PASS  Waiting for file changes...
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Checklist

• The change is focused and includes tests for behavior changes.
• Relevant tests and the build pass locally.
• Public API, behavior, or user-guidance documentation is updated where needed.
• Accessibility impact has been considered, including keyboard navigation and focus visibility.
• This pull request does not disclose a security vulnerability.

[alekswebnet]

Released in v4.3.0.

Thanks @nvi9, both for the fix and for the test that toggles the media rules directly instead of depending on the host's theme. That made it straightforward to build on in #60, where the guards now match a data-color-scheme attribute the element sets, so the theme no longer depends on inline-style text at all.