Use custom properties for color variables

[tvdeyen]

What is this pull request for?

Instead of using Sass variables we use custom properties for all colors.

This also removes the usage of any Sass color functions.

Checklist

• [x] I have followed Pull Request guidelines
• [x] I have added a detailed description into each commit message

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 95.96%. Comparing base (5138585) to head (7c73a9d).

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #2938 +/- ## ======================================= Coverage 95.96% 95.96% ======================================= Files 232 232 Lines 6272 6272 ======================================= Hits 6019 6019 Misses 253 253 ```

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