I was really annoyed that my themes wouldnt work on iOS4 (just as everyone else probably is) I'm not sure if this fix is going to work for every though. One of the themes that i downloaded from Cydia worked somehow(i dont remember which) and i looked through the properties of the files in the working one and compared it to the ones which didnt work. I found that in iFile, when u click on the blue arrow next to the folder name, and scroll down to the ownerships, the working theme had 'mobile' for both the 'owner' and 'group'. iFile lets you change the owner and group for the folders, so i went through and changed all of the folders in file attributes to 'mobile' (it took a while). After that, the theme worked perfectly fine. Again, im not sure if this will work for all the themes but it worked for me