I was mostly thinking about a way to provide custom CSS in the settings, either copy/paste or upload a file. If those style rules was loaded at the end of the cascade, it would make it easy to "hack" together your own styling that would be persistent across browsers and devices, easy to share, and could be changed on the fly.
I would prefer not to use add-ons for styling. It is possible (quite easy in fact) to write custom CSS for a domain with Firefox. The problem is that these changes would be limited to that browser.
It is a great idea to make the UI as minimal (and functional) as possible, akin to HackerNews! Sane defaults and simple html structure lends itself well to (optional) personal customization. I know this has been a big thing for many subreddit's, where every sub can load their own css.