I like the principle that I ought to be able to reach into some of the innards of anything that supports reflowing text.
I recently made a user style for the agora because I thought it'd be a fun use for Fern Text and also the light theme wasn't loading anyway.
One fun thing is that you don't have to worry about font size when they're not going over the network, so you can use whatever typefaces you have on hand.
The spirit of user styles extends beyond fussy Firefox application data folder CSS, though. My "I hate you for not valuing my comfort enough to style this" bookmarklet for unstyled HTML has the same energy.
javascript:(function()%7Bbs%20%3D%20document.getElementsByTagName(%22body%22)%5B0%5D.style%3Bbs.maxWidth%20%3D%20%2280ch%22%3Bbs.marginLeft%20%3D%20%22auto%22%3Bbs.marginRight%20%3D%20%22auto%22%3Bbs.lineHeight%20%3D%20%221.5%22%7D)()