Site dark mode

[osfanbuff63]

This might not be possible with the current setup but would love a dark mode for the wiki. It might need to wait for
Fabulously-Optimized/fabulously-optimized.github.io#1, since in theory it wouldn't be too hard to move everything over to that site in a /wiki/ subdirectory or something. But that's a project for the future :P

[Madis0]

I wish there was a toggle, but that's on GitBook. https://docs.gitbook.com/spaces/customizing-a-space#theming
Right now there is just either light or dark, and I think light is more readable.