To get the themes badck you'll have to reinstall iPhoto as follows: delete the current application and all files with "iPhoto" in the file name with either a .PKG or .BOM extension that reside in the HD/Library/Receipts folder and from the /var/db/receipts/ folder,
Then install iPhoto from the source it came on originally.
I had this problem with iPhoto 11 and Lion 10.7.4. I re-installed from the iLife 11 DVD and that fixed it.
Note - after the first re-install, the iPhoto app appeared with a white cross over it and wouldn't launch ("damaged"). I moved the application to the trash, then ran the install again. I had to download a couple of software updates before the thing would run, but that solved it.
Note - the problem occurred when wiped my HD and reloaded the system from a Time Machine backup, don't know if this caused the problem or not.