Finally I did it Take a look: Custom Theme Video

Is is actually possible to replace the original files or to copy the original theme to a new folder and to create a 'new' theme?

Hi guys, I've put my own music files in OGG format into the new music folder (Recalbox 4.0.0) on the share partition and named them the same like the emulators, but they don't play in ES... Still the original background music... Now I would like to replace the music on the original theme, but I've read, that the file system is actually read only. How can I insert my own music files?