__main__: Implement basic darkmode provisions

[irgolic]

Description of changes

• Sets 'darkMode' property in QApplication if 'dark' is specified in stylesheet. This can be easily queried globally elsewhere to color decisions
• Sets default pyqtgraph colors from QPalette on startup

Also removed the pyqtgraph colors set on owpalette.py import (please don't call code like this :x)

This is the first step toward proper dark mode support. Most (if not all) visualizations hardcode their colors. These hardcoded colors need to be removed and delegated to global default color settings (as set in 8f92990).

Note, this only works on startup, if the OS changes its color scheme (as Mac can upon sunset/sunrise), it won't update accordingly. But it's a first step.

This was transplanted from #5287. That PR will change into setting new default colors.

Includes

• Code changes
• Tests
• Documentation

[codecov]

Codecov Report

Merging #5526 (6f58628) into master (aa628bf) will increase coverage by 0.00%.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master    #5526   +/-   ##
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  Coverage   86.36%   86.36%           
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  Files         304      304           
  Lines       61665    61663    -2     
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+ Hits        53254    53255    +1     
+ Misses       8411     8408    -3