Btw, I probably should have mentioned other examples too, not just XenonPH's work. If you google "VP" or "FP" plus "reskin" you'll find a wider array of examples. Also check out Iceman's stuff right here:
pinballnirvana.com
In every case, he started with a table with a different theme.
Dubious, I think the best way to start this project is to go in to either VP or FP, load up a table, and then look for the images / texture manager. Scroll through the list checking them out, and save one or more locally to your machine. Then open up a graphics editor, like Windows Paint, GIMP, Photoshop or whatever.
Make some changes to the graphic, save, and then reimport that image in to the pinball editor. Rename the original version of that graphic so that it ends in ".bak" or something, then change the graphic you imported to the original one's name. Run the table again, and now you should see your modifications on display.
This is basically how you change (or "reskin" or "re-theme") a table's graphics to something fresh. Just that you'll eventually want to do it with most of the table's images, not just one.
Next step would be doing something similar with the sounds, but this is the best place to start I can think of.