There're some extra features (e.g. bookmarks) that utilize scripting and data storing. So a userstyle wouldn't cut it. A user script though would. Also privacy-wise I think there isn't a way to limit an extension to a single site, something possible with a user script.
Yes, you're right on asking permissions. I didn't had a site-specific extension so couldn't see them. Unfortunately restricting any extension, if it doesn't restrict itself, is only possible on Chrome and its derivatives.
