I have "bricked" my 3DS while playing with themes !
I don't know how, but one of my theme was corrupted. It could be opened by YATA, but one image wasn't the right size (border images, you didn't say what size they have to be) and the theme was succesfully installed, but 3DS WON'T boot ! If I remove the SD card, it will load the default theme and boot, but when I replug my SD card, hop, 3DS don't boot! To solve that I took another SD card, let the 3DS save the default theme on it, put both on my computer and copy the files to my original SD card
I only lost my icon arrangement
)
Sorry for my English, it must be annoying to read me u_u