[WIP] Dark Mode

[mfuerni]

Implement dark mode (dark and light color scheme) based on Bootstrap color modes. Add color scheme switcher to top app bar.

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[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 10.09%. Comparing base (0dcd2eb) to head (00a2fd6).

Additional details and impacted files

@@            Coverage Diff             @@
##             develop    #6069   +/-   ##
==========================================
  Coverage      10.09%   10.09%           
  Complexity      7483     7483           
==========================================
  Files            725      725           
  Lines          23997    23997           
==========================================
  Hits            2422     2422           
  Misses         21575    21575           

Flag	Coverage Δ
behat	0.00% <ø> (ø)
phpunit	10.18% <ø> (ø)

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