docs: add missing dark mode back

[bearomorphism]

The original config seems to be incorrect. The dark mode toggle button does not show.

Reference: https://squidfunk.github.io/mkdocs-material/setup/changing-the-colors/

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 97.88%. Comparing base (120d514) to head (033ddd9).
⚠️ Report is 881 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #1718      +/-   ##
==========================================
+ Coverage   97.33%   97.88%   +0.55%     
==========================================
  Files          42       60      +18     
  Lines        2104     2605     +501     
==========================================
+ Hits         2048     2550     +502     
+ Misses         56       55       -1     

Flag	Coverage Δ
unittests	97.88% <ø> (+0.55%)	⬆️

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