Xperia Themes uses themes that are already available via Google Play. To find and install new themes, go to the Themes menu on your Xperia handset and press the + button in the upper right corner. After that, you’ll be able to see all the available themes and download them. All in all, this seems like a pretty cool way to make your Xperia UI look just the way you want.
A video presenting Xperia Themes can be watched below: