Yeah, there are extensions that enable injecting custom CSS. I'm using Stylus in Chrome (switched to that from Stylish about two years ago) and essentially you need to override the native CSS with lots of !important style declarations. Basically like Inspect Element but will load every time once the relevant website(s) is done loading.

If the HTML classes and ids are straightforwards that's fairly easy, like old.reddit for instance. But every time they change the classes you need to go in a manually tweak it. And once a site starts obfuscating their code it's not worth the effort anymore.
But it's possible and for a while I honed my meager CSS skills by doing my own bespoke stylesheets. :)