Need help making a "theme gallery"


I’m on a mission, and I need some help. I want to make a “theme gallery” (take a look at to see what I’d like to recreate. Essentially, I’m hoping for a system that builds a gallery of thumbnails from all my themes, allows people to get more information about the theme, see a bigger version of the thumbnail, or activate the theme in a preview mode.

This seems to be pretty standard functionality out in the Interwebz, but I’ll be danged if I can find instructions for how to build it. I’d really appreciate any help anyone can send my way.