Menubar: Added theme.menubar_bg_base

[SuperDuperDeou]

Adding the theme.menubar_bg_base to lib/menubar/init.lua, this allows for the base widget to be drawn with a different background color than the items. This is especially useful with semitransparent themes to avoid unintentional alphas.

[Aire-One]

@SuperDuperDeou can you please rebase for CI checks to pass?

[codecov]

Codecov Report

❌ Patch coverage is 0% with 1 line in your changes missing coverage. Please review.
✅ Project coverage is 54.43%. Comparing base (d37e3c6) to head (dd3e9cf).

Files with missing lines	Patch %	Lines
lib/menubar/init.lua	0.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #4010      +/-   ##
==========================================
+ Coverage   54.25%   54.43%   +0.18%     
==========================================
  Files         261      263       +2     
  Lines       31173    31218      +45     
  Branches     1143     1143              
==========================================
+ Hits        16912    16993      +81     
+ Misses      13751    13715      -36     
  Partials      510      510              

Files with missing lines	Coverage Δ
lib/menubar/init.lua	16.45% <0.00%> (ø)

... and 11 files with indirect coverage changes

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.