I have no interest in ever using them, although I do use iPhoto to store my photos. Will deleting the entire Themes folder mess anything up?
I found if you leave all the folders starting with Printing... and the Assets folder that are in the
/Library/Application Support/iPhoto/Themes folder, you can still print.
Just delete all the other folders and you will regain a chunk of disk space.
You won't be able to print, so bear that in mind.