When I first bought my copy of one of my favorite games of all time, I had no idea I would be the first person ever to actually mod textures into the game.
It's honestly kinda sad. I posted this a little while back on the sub reddit, but I thought I might as well post it here too.
I hope this brings attention to the fact it's possible, as I really hope other people mod this game to.
Here's how to do it:
Download 3 the tools that are necessary for this mod, GARCtool (used for extracting and the recompressing files), Ohana 3ds rebirth (not technically necessary because it doesn't have the ability to edit anything, but it's the only tool that allows you to view the actual model with it's textures before you open it in the game) and Kuriimu THE ORIGINAL; not Kuriimu 2 (this allows you to export and then reimport textures).
After you have your dumped game, choose the model you want to modify (this model can be found in (models rooms room_room100_LZ.bin). make a copy of this file (because it's just a good idea) and then open up GARCtool, choose open file, select that file, and then hit process (do not click "delete compressed file") the file should be called dec_room_room100_LZ_CGFX.
Open file with Ohana3ds to make sure that it's working properly.
Open Kuriimu, the go to the top right corner of the screen and open Kukkii. Open up the extracted file and then export the text that you'd like.
Import your new texture.
Open Kukkii, reimport over the old texture your new one, and save.
Open Ohana3ds to make sure it looks like it's supposed to.
Open GARCtool, click the dec_room_room100_LZ_CGFX and then click process, this will recompress the file.
It's done! Just load the mod into the game!, how you're supposed to do that depends on whether or not your using and emulator or a hombrewed 3ds and what type of emulator or homebrewed 3ds you are using.
I'm looking into to better methods for doing this as I've noticed a few bugs (somtimes the old textures peak through a little bit) and I'm working on importing new models to create completely custom rooms.
From what I've seen, I haven't found any other mods like this, and if there are, then I'm really glad I'm not the only one who bothered to mod this game.