Convert scss variables to css variables

[nothingrandom]

Fixes #14085, moving towards mozilla/addons#2193

This work was split out from #14244

Refactors the codebase to use CSS custom properties for colors, making the UI more themeable and consistent.

[diox]

prettier is complaining

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 98.12%. Comparing base (fd38611) to head (41e8a67).
⚠️ Report is 16 commits behind head on master.

Additional details and impacted files

@@           Coverage Diff           @@
##           master   #14252   +/-   ##
=======================================
  Coverage   98.12%   98.12%           
=======================================
  Files         266      266           
  Lines       10647    10647           
  Branches     3285     3285           
=======================================
  Hits        10447    10447           
  Misses        187      187           
  Partials       13       13           

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