Fix repo sub_menu font color in arc-green

[jolheiser]

As discussed in Discord, this PR fixes the coloring of the active tab in a repo.

Before

After

As for actual changes, I opted to add the ui class to the link, which breaks it from the CSS that styled it black beforehand. Perhaps an explicit override in arc-green.css would also make sense, but this seemed to be the simplest fix without altering existing styles.

[codecov-io]

Codecov Report

Merging #6477 into master will increase coverage by <.01%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master    #6477      +/-   ##
==========================================
+ Coverage   39.39%   39.39%   +<.01%     
==========================================
  Files         393      393              
  Lines       53271    53271              
==========================================
+ Hits        20986    20988       +2     
+ Misses      29299    29297       -2     
  Partials     2986     2986

Impacted Files	Coverage Δ
models/repo_list.go	67.89% <0%> (+1.05%)	⬆️

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