Menubar themes

[JessToudic]

Fixes #156.
Changed styles in the menubar to themes, so that numbers, markers and buttons change together.

[codecov]

Codecov Report

Merging #157 (3ab5d49) into dev (990288b) will increase coverage by 0.19%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##              dev     #157      +/-   ##
==========================================
+ Coverage   74.69%   74.89%   +0.19%     
==========================================
  Files          29       29              
  Lines        3154     3151       -3     
==========================================
+ Hits         2356     2360       +4     
+ Misses        798      791       -7     

Impacted Files	Coverage Δ
minegauler/frontend/main_window.py	51.11% <100.00%> (+0.94%)	⬆️

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