[High Contrast Mode] Allow forced high contrast themes to also force color mode

[cee-chen]

Summary

Note

This PR merges into a feature branch.

The impetus for this PR stems from this earlier conversation: #8193 (comment)

Essentially, our use of forced-color-adjust: none can cause visibility issues with fullShade/emptyShade, if we don't respect high contrast themes that force light/dark mode.

At this point, we need to treat forced colors/high contrast themes as superceding any application setting (since they do in reality) - hence this architectural change.

QA

• Go to https://eui.elastic.co/pr_8200/#/theming/high-contrast-mode
• Use Chrome/Edge devtools > rendering to emulate both forced colors and prefers color scheme media queries
  
• Confirm that toggling the color scheme correctly updates the color mode, and that EUI's theme switcher toggles in the nav are unable to be toggled

General checklist

• Browser QA
  • Checked in both light and dark modes
  • Checked in both MacOS and Windows high contrast modes
    • (emulate forced colors if you do not have access to a Windows machine.)
      - [ ] Checked in mobile
      - [ ] Checked in Chrome, Safari, Edge, and Firefox
      - [ ] Checked for accessibility including keyboard-only and screenreader modes
• Docs site QA
  • Added documentation
    - [ ] Props have proper autodocs (using @default if default values are missing) and playground toggles
    - [ ] Checked Code Sandbox works for any docs examples
• Code quality checklist
  • Added or updated jest and cypress tests
    - [ ] Updated visual regression tests
• Release checklist
  • A changelog entry exists and is marked appropriately.
    - [ ] If applicable, added the breaking change issue label (and filled out the breaking change checklist)
• Designer checklist - N/A

[mgadewoll]

🚢 🐈‍⬛ The updates works as expected and code LGTM 👍

[kibanamachine]

Preview staging links for this PR:

• Docs site: https://eui.elastic.co/pr_8200/
• Storybook: https://eui.elastic.co/pr_8200/storybook
• New docs: https://eui.elastic.co/pr_8200/new-docs

[elasticmachine]

💚 Build Succeeded

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• Commit: f1f7708

History

• 💚 Build #3172 succeeded faa1510