Switch CSS color variables to use relative colors

[Ernest314]

See: https://stackoverflow.com/a/71098929/1292093 (CSS Color Module level 5)

This is used for .circle-flip-box in index.html's background-color.

[Ernest314]

Also see:

• https://caniuse.com/css-lch-lab
• https://www.w3.org/TR/css-color-5/
• https://github.com/web-platform-tests/interop-2022/
• https://bugzilla.mozilla.org/show_bug.cgi?id=1352757
• https://bugs.chromium.org/p/chromium/issues/detail?id=1026287

[Ernest314]

LCh color picker: https://css.land/lch/