Make work with dark mode, high-contrast, forced-colors

Problem/Motivation

When using dark mode, high contrast mode and force-colors, the borders and spinner appear in grayscale/reduced opacity. They should remain solid white to improve visibility and respect the high contrast setting.

Steps to reproduce

1. Use a chromium based browser (we tested on Microsoft Edge on Mac, also Chrome)
2. Open the "Rendering" panel in the dev tools
3. Check "Enable automatic dark mode"
4. Set forced-colors: active
5. Set prefers-contrast: more
6. Observe the Project Browser main page at /admin/modules/browse
7. Borders, and spinner appear in grayscale/reduced opacity

Screenshot: https://www.drupal.org/files/issues/2023-06-21/Screenshot%202023-06-21%2... →
Spinner video: https://www.drupal.org/files/issues/2023-06-07/highcontrast.mp4 →

Proposed resolution

Popular sites don't seem to have this issue. Update the CSS to make sure the high contrast settings are respected.

Remaining tasks

• ✅ File an issue about this project
• ☐ Manual Testing
• ☐ Code Review
• ☐ Accessibility Review
• ☐ Automated tests needed/written?