Feature/ Make tab bar border-bottom theme-able

[akinsho]

This PR is one of some upcoming which allows specific sections of oni's ui that currently default to other colors in the theme specifically themable, in this case the border bottom of the tab bar default to tabs.background meaning it can't be specifically styled or hidden which is less flexible. This PR adds a specific config for this setting but maintains the default of tabs.background if no specific key is specified so it is backwards-compatible

[codecov]

Codecov Report

Merging #2585 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master    #2585   +/-   ##
=======================================
  Coverage   45.26%   45.26%           
=======================================
  Files         361      361           
  Lines       14564    14564           
  Branches     1908     1908           
=======================================
  Hits         6593     6593           
  Misses       7747     7747           
  Partials      224      224

Impacted Files	Coverage Δ
browser/src/UI/components/Tabs.tsx	77.86% <ø> (ø)	⬆️
browser/src/Services/Themes/ThemeManager.ts	69.73% <ø> (ø)	⬆️

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[CrossR]

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