In Japan at the moment
If you’re really into 3DS themes and happen to live in Japan, you may want to check into this.
Nintendo is bringing out two new themes based on the new Volcanion and Magearna film, which you can find at various Pokemon Center locations and retailers. The exact list includesAeon Malls, Eidon, Ito-Yokado, Toys”R”Us, Bic Camera, Yodobashi Camera and Fuji stores. Serebii is reporting that you basically request a serial code, just like the Mew event last month, and you’re good to go.
It starts on March 18 and will run through April 8. There’s no word currently on if this will extend to the US, but you can probably bet on it.