Sony intros Xperia Themes for smartphones running Android 4.3 or later
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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.
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source: Sony
Things that are NOT allowed: