I think "Metamorph" in the app market is what you're looking for. It allows you to theme everything on your phone.. somehow. I couldn't figure it out. But then again, I only glanced at it and uninstalled it. I think you have to build the files/themes on your PC first and then import them with the app to apply them. But it may be a start for you .
Oh and you have to be rooted (which I would assume you are if you're asking how to do this).
Edit: Found this whole Metamorph guide for you on XDA's forum.
For Metamorph I have taken already made skins and modified the files within the package....I think its a good place to start.
The Metamorph packages are simply rebranded zip files so they are easy to open up and edit....just look for all the image files