💄 Fix light/dark themes for admin

[TimdeBeer1]

Fixes #maykinmedia/open-api-framework#149

Changes
Fixed light/dark theme in admin UI with the missing toggle.

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 94.66%. Comparing base (729fce6) to head (58f9413).

Additional details and impacted files

@@           Coverage Diff           @@
##           master     #616   +/-   ##
=======================================
  Coverage   94.66%   94.66%           
=======================================
  Files         148      148           
  Lines        5171     5171           
=======================================
  Hits         4895     4895           
  Misses        276      276           

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