Akez has released a “PSP Theme Changer”.

Author release notes:

I created this homebrew app so i could flash my own waves to my PSP. I dont know whether there are other applications like this but i would still much rather use my own as i know there is nothing which harms my PSP. It is fairly easy to use and it shouldnt brick your psp unless the file you select to copy is not a system_plugin_bg.rco file.