Which is handy but not relevant to my question of "why can't an extension do this" with the implied "so I don't have to fiddle with copy-pasting text off the internet into configuration files I'm not entirely familiar with using." Not to mention that by default Firefox 69 disables loading of userchrome.css and you can bet your britches they're doing that in future anticipation of removing the feature entirely just like they did with everything else they've stripped out.