The README.md mentions that a background image can be applied, but changing the background color is very difficult with Wikipedia's CSS. As a workaround, you can use a base64-encoded image of a single black pixel, and ensure the "Background Image Type" is set to the default "Tiled".

Here's the base64 URL for the black background. Put this in the field for "Custom background; include url()":

url("data:image/gif;base64,R0lGODlhAQABAIAAAAUEBAAAACwAAAAAAQABAAACAkQBADs=")

It's not perfect, but it is black! 😉

I thought I'd leave this issue here in case it adds any creative ideas for touching up the README or future PRs. Perhaps there is a way to generate this base64 in a JS library based off of user-supplied CSS colors? 🔥