This extension is the best thing to happen to Firefox since Electrolysis and Quantum. I use it on, and it works well on, nearly every Web site. For the few sites that don't jibe with default settings, it manages per-site settings for custom colors or flat out disablement very elegantly. I can't give this enough stars.
UPDATE: I decided that this is the first browser extension worthy of giving the developer a donation for. And I did that. And I suppose I couldn't resist tooting my own horn about it. *toot*