Add dark mode

[hugovk]

This is based on the dark mode (https://github.com/hugovk/drop-python/pull/42) I added to https://hugovk.github.io/drop-python/, which was originally based on this repo.

Preview:

[meshy]

Apologies for not digging into this one yet.

I seem to remember having to avoid separate CSS files in SVGs in the past to avoid issues in some browsers, but I don't recall which ones.

Have you had the chance to test this on a range of browsers?

[hugovk]

Here's how it looks in some other browsers on macOS.

Safari

Firefox

Opera