docs: Use better aligned colors for dark mode

[stotko]

The dark mode of the Sphinx book theme has been modified since the port to Sphinx to reduce the excessively high contrast of the separator. and to make the background a bit brighter. To come up with better aligned colors, the contrast needs to be analyzed more thoroughly. Add a respective script for this purpose and define a better separator color while dropping the brighter background. Furthermore, prepare for using Sphinx Design and ensure consistency with these changes.

[codecov]

Codecov Report

Patch and project coverage have no change.

Comparison is base (d9a4587) 97.40% compared to head (549de19) 97.40%.

Additional details and impacted files

@@           Coverage Diff           @@
##           master     #397   +/-   ##
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  Coverage   97.40%   97.40%           
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  Files          31       31           
  Lines        2581     2581           
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  Hits         2514     2514           
  Misses         67       67           

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