Change the white background in webpages to another color

Problem

I'm currently using a dark theme in Firefox. It looks really nice, but many webpages use a plain white background. The resulting contrast is a little unpleasant and sometimes hurts the eye when I switch from a dark tab to a white tab. Is there a way to make firefox replace white backgrouns everywhere with some other color (light gray, for instance)? It could be a Stylish script, a userChrome.css hack, or anything that works (preferably as light as possible). To make myself clear: after I achieve my objective, the background color whenever I visit the Superuser site should be light-grey instead of white, and the same should happen to any other site with a white background (google sites, tech crunch, etc). Is there a way to do that?