Nice guide, working a Persona Q theme atm.
This might be a dumb question but are the BGM files saved outside of the "body_LZ.bin" file?.
my body_LZ.bin is 125kb which seems way too small to contain my bgm.bcstm file.
Going to test my theme if I can get tubehax running off my old 3DS.
Edit: Doh! posted in wrong thread. Can it be moved to https://gbatemp.net/threads/tutorial-creating-a-3ds-custom-theme-from-scratch-with-yata-v1-1.393602/
No, the background music is saved in bgm.bcstm but to use this file you must check "enable use of bgm" in the theme you are working on