@Kel - Yes, I did miss at first that the TweetPress theme does take the background image and colors from your Twitter profile directly through the API. The creator of the theme was quick to correct me on that. I also added an update to the post with that information (with a link to the comment).

Like redwall_hp said though, a number of other elements present in the theme are not provided by the Twitter API such as the logo (the TweetPress creator hinted he would be removing that in a future version) among other things.