https://www.reddit.com/r/PretendoHub/comments/1gjcwdp/did_pretendo_bring_back_the_theme_shop/ A similar reddit post.
Pretendo may bring back the theme shop, but I’m not one to know. Currently, Pretendo is focused on game server development and Juxtaposition.
A fan project:
https://gbatemp.net/threads/researching-how-the-3ds-theme-shop-works.630310/