You can access the different components of the theme through var/stash/themes. You can mix and match, twist and turn, do whatever you'd like to modify the image files as long as they're within reason. Meaning you'd have to have it under the same formats, under the same names, etc etc. Then again, you can just take parts of themes and just mash them together to create your own and stick it under the themes directory.
I'm taking the icons from Prestige HD and mixing it around with a few other ones that I like for their statusBar, dock, folder views, and notification mods. It takes a bit of fidgeting around if the theme does not incorporate one of the elements.