there's two folders to remove. also it won't actually remove installed themes, only reset the current used theme. how about this?
Q: CHMM2 crashed during theme installation, and when I reboot I get a black screen. What do I have to do?
A: Go on a computer and insert your SD card. On your SD card, go to "Nintendo 3DS/your_id1/your_id2/extdata/00000000/
archive". your_id1 can be anything. If there's multiple, try opening each one to see which might be yours.
archive will be two folders depending on your region. Delete these two folders.
- USA: 000002cd and 0000008f
- EUR: 000002ce and 00000098
- JPN: 000002cc and 00000082
This will erase your home menu layout (so you should save it in Home Menu Settings next time), and wrap all your icons.