Nintendo of Europe revealed that a bunch of new Sega themes will be available in the 3DS eShop this Friday the 13th.
Shown in the image accompanying the announcement tweet are Master System, Mega Drive, Game Gear, and Sega Mark III themes.
We previously caught sight of the Game Gear theme as Sega showed it off on the handheld console's 25th anniversary.
The theme turns the top screen into a Game Gear interface so that it looks like you're playing on one.
Presumably, the other themes do the same but for those other Sega consoles.