Well, CXMB for 5.03 GEN-A obviously doesn't support every theme once you convert it. Well, if you have apply a bad theme, your PSP will shut down, and you won't be able to boot back to GEN-A, because it tries to load the theme, freezes, and shuts off. This has happened to me a few times now, and learning from my own mistakes, I figured out how to remove a bad theme and fix your PSP so you can reload GEN-A. I think there's a guide on this somewhere else, but this is a guide in my words, and it should be helpful for someone.

1. Start the PSP up (just keep it in OFW 5.03) and plug it in to your computer.
2. Navigate to the theme that caused you your grief (navigate on your computer, obviously) and delete it.
3. Unplug your PSP and go into Theme Settings, and delete the Custom Theme.
4. Boot to ChickHEN.
5. Boot to GEN-A.

So far, it's worked for me every time. If you got a question, ask me.