You can still tweak Firefox internals but you have to resort to its native `userChrome.css` [0] since current extensions API does not allow access to every corner. Luckily you can use devtools for that [1] what makes it quite pleasant (at least you don't have to restart browser to see the change). You can use pre-made package ([2][3]) and toggle what you want.

[0] https://www.userchrome.org/ [1] https://www.reddit.com/r/FirefoxCSS/comments/73dvty/tutorial... [2] https://github.com/Aris-t2/CustomCSSforFx/ [3] https://github.com/Timvde/UserChrome-Tweaks

there's an extreme method of doing 'light text on dark background', like my guide below

http://unexistance.blogspot.com/2014/09/of-firefox-solarize....

in which all background & foreground color are of your choice. been using it for 5 years at least

Thanks, but I do like it to be easily switchable. Some pages expect certain background colors and changing them makes embedded svgs hard or impossible to see.

Maybe you can get away with using the solarized theme, but I prefer having a black background...

Thanks for this suggestions anyway

You can change the universal background color to anything :D that is just a guide

heck, I'm using a black background nowadays, trying to mimic blackboard color ha