I tried to install this a long time ago, but never got the Stylish Firefox addon to work. Now there’s a new version, that accommodates the recently added Search feature in Google Reader. So, I gave it another try, and I REALLY like it:
You can get the theme here, and it seems that you can install it without Stylish if you’re using Greasemonkey (see details at userstyles.org here). It’s just forcing replacement of the CSS stylesheet.
Give it a shot, and let me know if you like it!