Fans of new Kingdom Hearts heroes Terra, Aqua, and Ven can get a silver theme featuring all three of them. Square Enix is sharing it on the official Kingdom Hearts: Birth by Sleep site. Just click on download (left button) to get it.
There are other themes and signed posters available to Square Enix members, but you need to participate diary writing to earn them. Not really sure if you can sneak by on this, but you can try if you really want four more custom themes.