[stable8.0] 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 9.70%. Comparing base (caa947c) to head (0da33ae).
⚠️ Report is 2 commits behind head on stable8.0.

Additional details and impacted files

@@             Coverage Diff             @@
##             stable8.0   #4767   +/-   ##
===========================================
  Coverage         9.70%   9.70%           
  Complexity         266     266           
===========================================
  Files              124     124           
  Lines             6336    6336           
  Branches          1189    1189           
===========================================
  Hits               615     615           
  Misses            5599    5599           
  Partials           122     122           

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