What are some recommended sites to help build Twitter backgrounds?
This background would be used as a custom theme for a user's twitter page.
Note: Looking for web-app/service, not an app to download & install.
Well this is indeed a tough question to answer as the number of such websites that help you build twitter backgrounds exist in plenty now. Well here is a list of such websites that you can go through for a start: http://digitizor.com/2009/10/15/6-websites-to-find-awesome-free-twitter-backgrounds/