I've written a dark css theme for the website, as I prefer a dark background with light text when reading from a screen.

I'm sharing it in the hopes that others find it useful.

https://userstyles.org/styles/156189/dark-bigclosetr

For now I haven't themed the forums or story editor, just the parts needed for reading. I may well come back to the rest at some point as I'm currently writing a story of my own.