In the settings menu, there's a Theme option. If it's set to Cookies, change it to Original. 3.71 comes with Cookies, which doesn't appear in the themes folder.
If it's not that, check if you have a cxmb folder on your memory stick. If so, then the somebody that installed CFW also installed cxmb. You can disable it in the M33 recovery menu.
If it's not that either, then he/she may have installed the custom theme in the psp's flash. At that point I wouldn't know how to revert it if you don't have the original files to replace the custom ones.