Yeah, search 0004008c00008f01 on the title key site.

They'll be deleted if you visit the eshop though.

 

3ds title key site is the only hint I can give because it's piracy related. Click on the qr symbol beside the title id I mentioned above and install it with FBI's remote install option.

 

Go to your theme selection menu and choose a new theme.

 

There was once a way, by making the CIA yourself. You may probably still find it on 'that iso site', it consisted of all themes in separate files and a script to build the cia, which you had to feed the IDs of the themes.

 

Another way, which is recommended by many, is to use CHMM. May be homebrew and the themes won't show up in the 3DS's own theme picker, but those are easier to manage (just plop the theme files onto the SD card and choose it in CHMM, no need to rebuild and reinstall a CIA) and are resistant to launching eShop, as they are not DLC but extra data on SD card.
Official themes dumped into CHMM format can be found, you guessed it, on 'that iso site'

 

Oh, definitely, didn't know something new aside from CHMM was released.

 

You may want to delete the link, links to copyrighted materials like that are forbidden.