I'm not big on Firefox extensions. It's not that I don't like them, I've just never spent a lot of time fooling with them. I have Greasemonkey installed, but don't do much with it. I've also got Google Notebook installed which I use from time to time. But upon reading a digg link today I spent a few minutes fooling with an extension called Stylish. It seems there's a whole crowd of people who want to skin different pages (or all pages) with different themes.
I tried out a few Google Reader themes, but actually came to the conclusion that I like the stock gReader theme better than any of the several I tried. I've said before that I like the functionality, simplicity and look of Google's Gmail, Reader and Calendar. I'm not sure I really want (or need) to theme them. But in case you're into that type of thing, you should install the Stylish extension and visit userstyles.org.
[that being said, I see that userstyles.org might be suffering from the digg effect right now so be patient]