Well, there are over 100 themes at appearance themes, and many of the ones released over the last six months have customizable backgrounds and background colors along with some that let you determine whether the sidebar is on the left or right, or both. Some have sidebars at the bottom instead of on the sides, and one even has them at the top.
The thing is, this is a modified multi-user version of wordpress with restrictions and features not found in the normal stand-alone version of wordpress, so themes from outside of wordpress.com have to be modified to work here, and since we cannot upload themes, it simply isn't possible. Each theme introduced here has to be inspected very carefully for security issues and then it has to have all the stuff necessary to support the features and restrictions here added or in some case taken out of the theme before it can be introduced. They also have to go through testing with all browsers and all versions to make sure they are substantially bug free before release.
If you want a theme from outside wordpress, then you will have to have the necessary expertise to set up and manage a website on a third-party web host and will be responsible for all installations, all upgrades, all backups and all troubleshooting.
If you have CSS editing experience, a lot can be done with the themes here with the CSS upgrade, but to really trick things out, you need that experience. There is no official support for the CSS upgrade and only a couple volunteers here that answer CSS related questions.