You can only install an XMB theme if you are using firmware 2.00 or greater. The .p3t files, must be stored in the "PS3/THEME" director of a memory card or external hard drive. To install the themes, go to [Settings] > [Theme Settings] > [Themes] and click install. Select the device on which the themes are stored and select the theme you wish to install.
Click on an installed theme to set the theme, delete the theme or view information about the theme.