I think I already got the right answerFrom reading his post, I don't think he actually made a theme package, I think he may have just loaded it with the extdata tool or Custom Home Menu Manager.
The theme doesn't need to be in a package to be loaded correctly, it just has to be the LZ_body.bin file and the bgm.bcstm file.
EDIT: I don't know for sure, now I'm getting confused lol
Only thing left to ask is how will I do it so that I can change to the custom themes with only a few clicks using the "Change Themes", in other words... do what Reisyukaku did: https://gbatemp.net/threads/tutorial-how-to-make-a-custom-3ds-theme.377533/#post-5242620
Or is there a thread how will I use the Custom Home Menu Manager?