Keep in mind, Kivy tries to remain platform agnostic. Interacting with the "background" is usually quite platform specific (e.g. changing the background image involves different actions on Android, iOS, Windows, Linux and MacOS). Consider also, it's probably not a good idea to try to do that. Users have a known and predictable way to change the background image Now a new app comes and does it differently?
So, you could write platform specific code to do that, but to be honest, it does not sound like a good idea. Each platform has it's own way to do that. And when you install an app, you (usually) do not want it to change your background. OIr am I misunderstanding something here?
Thanks
ZenCODE