There are hundreds of websites available which offer you some nice Twitter themes and backgrounds for free but most of them let you download the theme first which a user have to upload to its profile.

This site is quite remarkable as it provides you a realtime theme preview - you can have look before setting up on your profile as how a theme really look like on your current resolution.

Tweety Got Back is an amazing site which offers you stylish backgrounds, themes, color combination  to set your Twitter profile like never before.

On start-up you are presented with some random themes. Use categories to browse themes of your interest.

In-short - All you have to select a theme of your choice and sign-in with your twitter account and voila ! - the new theme will be on your profile.