most of xmb customization file located in flash0:/vsh/resource. this folder is not critical. u will only experience half brick if u messed up with any files in the folder (u can recover it with recovery menu)...
-a little advice before customizing your psp:-
1. make a free space in flash0 cus most of edited files will use more space than available. u can delete
gb3s1518.bwfon &
jpn0.pgf
located in flash0:/font and replace it with a copy of ltn0.pgf.

2. do not change the files in the critical file/folder for example flash0:/kd (sorry i forgot the actual directory, u can look around and search which one is the critical folder). changing/replace files in this folder will cause u a fully brick psp