I found out around a year ago when I hacked my 2DS that there's a way to make custom 3DS themes. I already have some themes I bought from the Theme Shop in years past, but I wanted to make one of my own. My laptop at the time was too outdated to download any of the suggested programs, so I had to put my idea on the back burner. I got a new laptop recently, so now I can try it for myself. (Though I only had the option of one program, since it's the only one that works for Mac.)

I based the theme around one of my current favorite games, Twisted Wonderland. It took me about 2 days to set up. There was definitely a learning curve, but I was starting to get the hang of it as I went along. I used some art from the game itself for the top and bottom screens, and I made the button colors and folder designs similar to the featured character's outfit.

The hardest part to set up was the audio. For one, I wanted to include a full song from the game as the background music, but it's nearly 3 minutes long. The maximum file size they allow for the BGM is 3.3 MB, so I had to compress the hell out of it and it sounded so bad. I had to compress it from about 44,000 hz (standard) to 16,000 hz. I know the 3DS speakers aren't the best, but it sounded like it was playing underwater. So I had to cut my losses and stop it after the first chorus. I faded it out a bit so the loop isn't as jarring. 

Even when I had the audio files in the correct .wav format and 16-bit encoding and stereo sound that it required, it wouldn't let me preview or change the files once I uploaded them. Those options were grayed out. I spent so long digging on Google and Reddit threads for help, but found couldn't find anything helpful. Anyone who had the same problem as me got no responses to their questions. Eventually, I found an older version of the program I was using and downloaded that and the audio would play just fine. We're back in business baby!

I got the theme downloaded on my 2DS last night and it's actually PEAK. Here's what the theme looks like:

Honestly not bad for my first time ever using this program. The green isn't nearly as vibrant on my 2DS as it is in the preview, but it's still fine.

These are the folders I made. It's the same pattern as the inside of the character on the top screen's coat. Not sure why they didn't show up in the preview, but whatever.

And this is the background music. PEAK song btw. Again, I had to cut it after the first chorus because it was too long. I reeeeeeeeeeeally wanted to include the full song, but I'd rather have a shorter, better quality version than the full version sounding like crap.

https://youtu.be/G5vuldFF-HE

I submitted the theme to the Theme Plaza website and it's currently being reviewed. I don't know how long it will take since I've never done this before, but I don't think there are any issues that would cause it to be rejected. I'll share the download link as soon as (if) it gets approved.

Honestly I'm really happy with how this theme turned out. I've had this idea since I found out this was something you could do, and it's finally being realized. Now that I have a better grasp of the program, I want to try making another theme. I already have a character in mind hehe