Fix martor dark style

[int-y1]

part of #2035

• on the main site, martor preview follows light/dark mode (main site includes style.scss, and style.scss has light/dark martor)
• on the admin site, martor always uses light mode (see AdminMartorWidget for code)

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 47.67%. Comparing base (fd7fb05) to head (7a7c274).
Report is 39 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #2374      +/-   ##
==========================================
+ Coverage   46.76%   47.67%   +0.91%     
==========================================
  Files         251      253       +2     
  Lines       13317    13648     +331     
==========================================
+ Hits         6228     6507     +279     
- Misses       7089     7141      +52     

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