The easiest way is to use your computer (well, for me it's the easiest). You download the theme to your iPhone and then use something like iFunBox to transfer the entire theme to your computer. Then, open the theme (it's just a folder of pictures ... and more folders) and delete the images you don't want.
I do this all the time as I usually only want the icons, myself. Although I check every folder, I usually end up deleting everything but the Bundles folder (though you will find what to keep by trial and error). I then go through every folder in the Bundles folder and [pretty much] remove all but the app icon image. I keep a few things, such as badges, but most everything is deleted.
Oh, and make sure you keep the com.apple.mobileicons.framework folder (if there is one). That is what will theme the AppStore icons automatically.
It's a pain to go through each folder, one by one. But, IMO, it's worth it because then I can either mix and match my themes or just run a theme with icons.
Good luck.
Marilyn
P.S. I recommend making a copy of the theme (that's on your computer) before you start any deleting ... just in case you make a mistake.