Dark theme for documentation

[whitequark]

See preview.

Unfortunately this approach (using sphinx-rtd-dark-mode) has several issues:

• [ ] Parts of the theme are outright broken and unreadable (see below; these are examples from the preview link)
• [ ] No detection of user preference via media query
• [ ] The top left corner looks weird; needs more contrast for the search box, and a lighter background
• [ ] The fading is incredibly annoying (try clicking the light/dark mode a few times in a row)

I'm not planning on fixing this but I do welcome assistance.