I use the one that came with MH4U. Apparently there's going to be another free MH theme in May, so woohoo!
I don't think I'll ever buy one though, even if they are just a couple bucks. I'm only on the home screen for a few seconds anyways so I can't justify the price.
I have it on shuffle so all the ones i bought which are; Xenoblade Chronicles: Monado, Zelda: Dire Moon, ACNL: Bulletin Board, Stars: Yellow & Purple, Chao Theme, and Zelda: Two Worlds.
I, sadly, haven't looked into buying any themes. I've been boring with the standard black theme lol. Going to actually look into some themes now! Didn't realize how cool some were. Are some exclusive to the "New" 3DS's? I kinda am stuck with my regular XL and refuse to get a New 3DS until they make the regular size New 3DS available in the U.S. Even though it probably will never happen. D:
I like the large screen of the 3DS XL but I've also grown to hate it lol. Everything looks so fuzzy on it and it bugs me. Also, I would like a more portable, more discreet system. Kinda stick out in public with the enormous XL and it makes me feel more self conscious haha.
I Have the Primal Groudon/Kyogre, A Link between Worlds, and Skyward Sword on Shuffle right now, although I also want the Xenoblade, Mega Rayquaza, and Major's Mask 3D, but My college's wi-fi doesn't support 3DSes.
I bought the Halloween Animal Crossing theme and the Rayquaza theme. If Shadow's theme played his theme song I probably would have bought that one, but the Radical Highway song seems like a weird choice to me. I'd have more themes but I feel they are too expensive just for a background.