Fix colors

[hluk]

Fixes background of the FakeVim command line in some themes.

Adds support for setting colors in theme file by name, for example: edit_bg=white.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 78.98%. Comparing base (40ca066) to head (cb182da).
⚠️ Report is 2 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #3182      +/-   ##
==========================================
- Coverage   78.98%   78.98%   -0.01%     
==========================================
  Files         223      223              
  Lines       29829    29831       +2     
==========================================
+ Hits        23561    23562       +1     
- Misses       6268     6269       +1     

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