Instructions
Hold down the "Guide" button on your Xbox 360 controller to turn the system on. Sign in to your Xbox Live profile.
Navigate to the "My Xbox" menu and highlight your avatar, then press "A."
Highlight "Change Theme" and press "A." Scroll through the list of themes, then press "A" to choose one. This changes the images and colors that appear in the background of your friends list.
Change the theme, if you are using an earlier version of the Xbox Dashboard, by pressing the controller's "Guide" button. Select "Settings," "Profile" and "Change Theme," then choose your new theme.