Generally, you'd use "user styles" for that, e.g. with https://addons.mozilla.org/en-US/firefox/addon/styl-us/ https://addons.mozilla.org/en-US/firefox/addon/st

Generally, you'd use "user styles" for that, e.g. with https://addons.mozilla.org/en-US/firefox/addon/styl-us/ https://addons.mozilla.org/en-US/firefox/addon/styl-us/, such as anything from https://userstyles.org/styles/ https://userstyles.org/styles/ or just `* { color: black !important; background: white !important; }`. The problem is that while this works great for the majority of sites, it breaks the layout of some edge cases. For example, this also renders normally-transparent backgrounds as white and thus suddenly some elements are hidden. Writing global style rules that work everywhere is hard and afaik typically avoided in favor of site-specific styles. I'm currently attempting to create one anyway (win95 appearance).