Nintendo Japan has revealed the new Nintendo 3DS themes and Pokémon Mate (ポケモンメイト), Pokémon Mega Rayquaza (ポケモン メガレックウザ) and Pokémon Hanafuda (ポケモン花札) were announced. These themes cost 200yen each and are available for purchase as of March 11th, 2015 on the Nintendo 3DS eShop store.

Pokémon Mate (ポケモンメイト)

Features: The protagonists as well as N, Cheren, Furo, Kamitsure, Subway Masters Nobori and Kudari share the spotlight in this theme. They move side to side as the player moves menu screens.
Music: Black and White - Let's Go Together! (～いっしょに行こう！)

Pokémon Hanafuda (ポケモン花札)

Features: A luxurious playing cards design based on Pocket Monsters Red and Green that fill the screen.
Music: Heart Gold/Soul Silver - Dancers (～まいこはん～)

Pokémon Mega Rayquaza (ポケモン メガレックウザ)

Features: The theme focuses only on Rayquaza who takes up both screens and changes to a Mega Rayquaza background as players move screens.
Music: Omega Ruby/Alpha Sapphire - Battle! Super-Ancient Pokémon! (～戦闘！超古代ポケモン～)

Sources: http://www.nintendo.co.jp/3ds/theme/list/01049/index.html, http://www.nintendo.co.jp/3ds/theme/list/01051/index.html and http://www.nintendo.co.jp/3ds/theme/list/01050/index.html