[stable7.3] fix: improve expand button visibility in OrgChart dark mode

[backportbot]

Backport of PR #4756

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 9.93%. Comparing base (d79281b) to head (3762060).
⚠️ Report is 2 commits behind head on stable7.3.

Additional details and impacted files

@@             Coverage Diff             @@
##             stable7.3   #4766   +/-   ##
===========================================
  Coverage         9.93%   9.93%           
  Complexity         266     266           
===========================================
  Files              124     124           
  Lines             6182    6182           
  Branches          1429    1429           
===========================================
  Hits               614     614           
  Misses            5445    5445           
  Partials           123     123           

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