Intro: I was sick of that yellow background and I looked for a firefox addon to edit css, but I found an even better addon that SAVES your css edits and let you share them! (they even update automatically in all computers when the author updates it).

1a.install Stylish addon for Mozilla Firefox
https://addons.mozilla.org/en-US/firefox/addon/stylish/
1b.install Stylish addon for Chrome
https://chrome.google.com/webstore/detail/fjnbnpbmkenffdnngjfgmeleoegfcffe
2.install the style for the forums
http://userstyles.org/styles/50910/drunkduck-gray-forum-background?r=1310933356
3.try it out (go to a random forum thread or to your pq inbox). Remember that you can turn it off too!

EDIT:
The newest version of “Sytlish” addon (for Firefox 6) can be found here (at the top):
https://addons.mozilla.org/en-US/firefox/addon/stylish/versions/