docs: correct CSS variables issue link

[Gerrit0]

The current link references a completely unrelated issue, likely accidentally replaced when migrating to this repo.

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[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Comparison is base (20dd157) 97.07% compared to head (3d5186b) 97.07%.

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #560   +/-   ##
=======================================
  Coverage   97.07%   97.07%           
=======================================
  Files          54       54           
  Lines        5027     5027           
  Branches      648      648           
=======================================
  Hits         4880     4880           
  Misses        143      143           
  Partials        4        4           

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