Initi Creates released today the 2nd Azure Striker Gunvolt II Nintendo 3DS theme package which includes 2 new themes for your 3DS in Japan. You can buy them one by one from the Japan Nintendo eShop for 150 Yen (with tax) / theme price with the theme song (as BGM) of the game from Aoki Raitei. Both theme includes normal wallpapers and according to Initi Creates the next theme package release on September 14, 2016.
You can find the themes in here: