Fix dark mode detection on Linux/GNOME

[felipdsa21]

The current implementation does not read the color-scheme setting at startup

Fixes #7817

Testing strategy

Changes were tested by setting the theme using GNOME Control Center and opening KPXC to determine if used the chosen theme

Type of change

• ✅ Bug fix (non-breaking change that fixes an issue)

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 42.85714% with 8 lines in your changes missing coverage. Please review.

Project coverage is 64.44%. Comparing base (d181f80) to head (bc20982).
Report is 381 commits behind head on develop.

Files with missing lines	Patch %	Lines
src/gui/osutils/nixutils/NixUtils.cpp	42.86%	8 Missing ⚠️

Additional details and impacted files

@@             Coverage Diff             @@
##           develop    #8477      +/-   ##
===========================================
+ Coverage    64.41%   64.44%   +0.03%     
===========================================
  Files          339      339              
  Lines        43963    43974      +11     
===========================================
+ Hits         28316    28338      +22     
+ Misses       15647    15636      -11     

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