refactor: migrate CSS to nested syntax with PostCSS build pipeline

[Zeroto521]

No description provided.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 69.84%. Comparing base (91c4fa1) to head (0fcd6bd).
⚠️ Report is 1 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #124   +/-   ##
=======================================
  Coverage   69.84%   69.84%           
=======================================
  Files          52       52           
  Lines        2315     2315           
  Branches      531      531           
=======================================
  Hits         1617     1617           
  Misses        549      549           
  Partials      149      149           

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