fix(theming): Adjust dark high contrast to fulfill WCAG 2.1 AAA contrast

[susnux]

• ⚠️ this is chained on fix(theming): Adjust status colors and make dark theme fully accessible #42287 and must be merged first

Summary

This adds high contrast support for the theming util and unit tests and adjust the high contrast dark theme to fulfill WCAG 2.1 AAA color contrast for text.

Checklist

• Code is properly formatted
• Sign-off message is added to all commits
• Tests (unit, integration, api and/or acceptance) are included
• Screenshots before/after for front-end changes
• Documentation (manuals or wiki) has been updated or is not required
• Backports requested where applicable (ex: critical bugfixes)

[susnux]

/backport to stable28

[JuliaKirschenheuter]

looks great, thanks!

[susnux]

@JuliaKirschenheuter I adjusted all themes for the color-success and color-error and added a test case for the border contrast. Should be good now.

[JuliaKirschenheuter]

@JuliaKirschenheuter I adjusted all themes for the color-success and color-error and added a test case for the border contrast. Should be good now.

That works now, thanks a lot!

[backportbot-nextcloud]

The backport to stable28 failed. Please do this backport manually.

# Switch to the target branch and update it
git checkout stable28
git pull origin stable28

# Create the new backport branch
git checkout -b fix/foo-stable28

# Cherry pick the change from the commit sha1 of the change against the default branch
# This might cause conflicts. Resolve them.
git cherry-pick abc123

# Push the cherry pick commit to the remote repository and open a pull request
git push origin fix/foo-stable28

Error: Unknown error

More info at https://docs.nextcloud.com/server/latest/developer_manual/getting_started/development_process.html#manual-backport

[susnux]

/backport to stable28