I'm pretty sure I could write a C library that reads a Body_LZ.bin theme if I had the time, all the informations we need are here : https://github.com/exelix11/YATA-PLUSOnly if there are details available on how to read the file format and access the image and audio data. Also the theme colour data would need to be present, so they probably would not be compatible.
Though that may take a while to read the source code, I guess @exelix11 or @Reisyukaku could help us understanding how this format works if they have the time. Also the theme colour data could be "overwritten" if a native 3DS theme is selected, so that would not generate any incompatibilities. Right now I have a ton of projects to work on so I may dedicate only really a little time to this...