When you first add it on, some sights might bug out, but this issues goes away. Do NOT uninstall based on that. Dark Reader isn't just useful for reading, but also for every day browsing. White screens are awful to look at and they hurt your eyes. Get this extension to help with that!

J'aime beaucoup cette extension, elle permet d'avoir un thème sombre pour la quasi-totalité des sites. L'interface est particulièrement bien faite, et les réglages personnalisables sont appréciables.

Takes the plain white background of websites and automatically changes it to a dark grey/black to match Windows dark theme. Does not break pages with custom backgrounds. Works very well. Very simple idea. I don't know why this isn't a native FF feature yet, but it should be.

Finally there's an add-on for general well-being of screen-staring tribe aka programmers, designers, developers, researchers, editors and all those who spend good part of their day/night staring at computer screen(s). I just love it's customizability, especially different profile per website. It's highly recommended to protect your eyes in case of prolonged computer usage.