Prior to the Vita release, Sony Europe started the Inside PS Vita program. This program would offer gamers updates on the system and unique downloadable content. While it originally offered themes for Little Deviants, Littlebigplanet and Resistance: Burning Skies, it seems there now sending a Unit 13 code too.
Unlike the others, Unit 13 is a dynamic theme and also includes all prior themes. It’s hard to say if this will be true of all future codes, but it might be worth signing up just in case. If you would like to see the theme, you can find a video of it below.