(THIS WILL ONLY WORK ON A HACKED PSP)

Well if you ever wanted add a custom theme to your PSP safely then CXMB is for you here is the link to the pack of CXMB for 3.90, 3.71, and 4.01 m33:
http://www.mediafire.com/?sharekey=28967202e021cd27d2db6fb9a8902bda

INSTALATION
1. Extract the rar file according to the version that you have.
2. Open the CXMB for your version (the one you just extracted from CXMB's.rar) there will be three files: cxmb, PSP, seplugins. Drag those three folders to the root of your Memory card. (it may ask something stupid just press "yes to all"
3. Once its on you mem card turn your PSP FULLY OFF. and turn it on holding R to enter the recovery mode.
4. When in recovery mode go to Plugins -> and then enable cxmb.prx
5. now exit the recovery menu and your all set to get custom themes

NOTE: I AM IN NO WAY RESPONSEIBLE FOR ANY DAMAGE DONE TO YOUR PSP. NOTE NOTE: THOUGH YOU PROBABLY WILL NOT ENCOUNTER ANY ERRORS. NOTE NOTE NOTE: ALWAYS USE FULL VERSION INSTEAD OF LITE. NOTE NOTE NOTE NOTE: ALWAYS USE ctf THEMES NOT REGULAR SONY ONES AND THEMES ARE SPECIFIC BY VERSION!

Please if you have any questions just reply to this topic below and I will do my best to help you out