Add variable for controlling backtop colors

[megawac]

No description provided.

[mention-bot]

@megawac, thanks for your PR! By analyzing the history of the files in this pull request, we identified @afc163, @gyeates and @simaQ to be potential reviewers.

[codecov]

Codecov Report

Merging #6038 into master will not change coverage.
The diff coverage is n/a.

@@          Coverage Diff           @@
##           master   #6038   +/-   ##
======================================
  Coverage    83.3%   83.3%           
======================================
  Files         199     199           
  Lines        4666    4666           
  Branches     1350    1350           
======================================
  Hits         3887    3887           
  Misses        779     779

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