I've managed to make a userstyle for stylish which seems to work well on my FFv42 (latest). using the OP's v3.
I added commented descriptions so you can see what each of us has done, and added all this at the top so its easy for anyone to personalize the colours to their tastes.
- added link color for link, hover, active, and visited.
- added a body background grey which surrounds the forum posts.
- a little more invert tweaking on the main forum page logo.
There are a lot of lines which could be removed. I'm not so great at doing that. Maybe someone else with good css skills and go through and work that problem. 10989 lines to be precise. So a lot can be cut out like mattk50 says.
Just copy into a new Stylish page, preview, and then save. And edit as you like.
Its not perfect but its a dark theme that works well enough. (tested in FF v42 latest)
I uploaded the Stylish CSS (v003) to a dropbox - LINK (my next post below has latest v003a update)
See these screen shots below-