Adding support for dark themed OSes

[raivisdejus]

Fix for #922

[codecov]

Codecov Report

Attention: Patch coverage is 18.75000% with 13 lines in your changes missing coverage. Please review.

Project coverage is 82.18%. Comparing base (cf90334) to head (2e976e5).
Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
buzz/widgets/application.py	18.75%	13 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #926      +/-   ##
==========================================
- Coverage   82.40%   82.18%   -0.22%     
==========================================
  Files          85       85              
  Lines        4717     4733      +16     
==========================================
+ Hits         3887     3890       +3     
- Misses        830      843      +13     

Flag	Coverage Δ
Linux	81.76% <18.75%> (-0.22%)	⬇️
macOS	82.06% <18.75%> (-0.22%)	⬇️

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