Stole some CSS from a dark themed discourse (teksyndicate in this case) and quickly threw it into a userscript. It has some stuff that's not necessary and its not perfect, but it works pretty well if you really want a dark theme right naow. Most of the issues with it could probably be sorted with some effort, and the css needs to be cleaned up since it has a bunch of references to a different website. cough. they probably dont mind. It might have been better to just do this from scratch but discourse's style sheet is a 3000 line monster and intimidated me way too much for that route.
To use it you need the userscript add-on greasemonkey if you are on firefox, or tampermonkey on chrome. Click this and you should get a prompt asking you to install it, if not you can download the file and install it manually through your add-on. Happy dark browsing! And, let me know if this even works. I haven't really thrown together any userscripts anything like this before.
edit: Updated to version 3, all of the really obvious issues are fixed
@JonnyRedHed put together a raw css version that will work css specifc addons (don't use stylish anymore, it was bought out and is now spyware), if you prefer it. For Firefox and For Chrome