It seems like the closest thing to this idea currently is Firefox CSS. [link] Honestly, if they would just officially add support for it (show it in the store), I would just use that.
I remember going through some bugzilla threads in which developers were concerned about the security aspects(which is really really huge and serious in this case) of this change. They are aware of this feature requests by the users. But they haven't reached a solution for this (yet?).
I support some form of official support for CSS themes. It would be safer anyway if they were vetted than Mozilla than downloaded from random third-party websites.