On Youtube, there is this nice button (easy to overlook - top left of the video) which lets one "turn off the lights": the site background changes from white to black, the text color changes from black to grey. There is an unrelated plug-in for Firefox called "Turned Off The Lights", which has a very similar functionality.

This makes websites so much easier to read. However, both technologies only work on YouTube. Is there anything to achive the same effect for all websites? Preferably with Firefox?

I.e.: I want to have very dark background and light text color on all websites viewed with Firefox, how can I do that?