People have suggested extensions here, which is fine, but Firefox also supports this natively via custom css files that it can load for you on startup. They can be used for both styling Firefox itself and for modifying websites' styling.

https://superuser.com/a/319322/1173126 (be sure to read the comments of that answer too, these days you need to switch a flag in Ff settings to enable this feature)

You can use Stylus (https://addons.mozilla.org/en-US/firefox/addon/styl-us/) to add custom CSS to websites.

On Chrome, I use an extension called 'Stylebot'. Not sure about other platforms.