Instructions
Turn on your PlayStation 3 and click on your username.
Scroll over to "Settings" by using the joystick on your controller.
Select "Theme Settings" by pressing "X" on your controller.
Select "Theme."
Choose which theme you want to use. Note that only some themes are capable of rotating. The best way to figure out if your theme can rotate is to select it once, make note of the background displayed and then select it again. If a different image is displayed, the theme is capable of rotating.