Adjust base CSSs and minimal theme to use Bootstrap variables

[ekdahl]

• Adjust minimal theme (and other base CSS files) to use CSS variables from Bootstrap instead of hardcoded colors, radius, thickness etc.
• This makes the minimal theme usable with both light and dark bootstrap themes, as implemented in Add light/dark mode switching #5256
• Next step would be to implement light/dark switching in the minimal theme.
• To test dark mode, add this attribute to the html element: data-bs-theme="dark"

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 35.39%. Comparing base (b5c8e72) to head (8ba1697).

Additional details and impacted files

@@            Coverage Diff            @@
##               main    #5328   +/-   ##
=========================================
  Coverage     35.39%   35.39%           
  Complexity    11182    11182           
=========================================
  Files          1165     1165           
  Lines         47998    47998           
=========================================
  Hits          16990    16990           
  Misses        31008    31008           

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.
• 📦 JS Bundle Analysis: Save yourself from yourself by tracking and limiting bundle sizes in JS merges.