Dark mode theme for documentation

[jcapriot]

This modifies the CSS sheet to do some fancy dark mode formatting of colors, and images to make things look nice in dark mode!

[codecov[bot]]

Codecov Report

Merging #292 (9a08fbb) into main (7aa4132) will not change coverage. The diff coverage is n/a.

@@           Coverage Diff           @@
##             main     #292   +/-   ##
=======================================
  Coverage   85.68%   85.68%           
=======================================
  Files          38       38           
  Lines       11026    11026           
=======================================
  Hits         9448     9448           
  Misses       1578     1578           

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