Interesting. I was looking in the package for the size of slide I was using; then it occurred to me to look in the other sizes ... and *those* packages have the graphic file!
So - once I've made the adjustments I want to the image and re-size it, I should be able to save it to my hard drive and then use the Image Fill ... Choose option to import it. Is that correct?
I don't know about Keynote's scaling for different sizes, but I also did notice that the package contents no longer "show" the theme's background image and other elements. I seem to recall these elements were visible (and extractable) in earlier version of MacOS.
But here's how to get those background files so you can change them to your liking. Just export the themed file to Powerpoint, then reimport the file to Keynote. Open the package of the re-imported Keynote file and you should see all of the elements that were previously unavailable. Worked for me.
I imagine these elements are just set to invisible these days, so maybe looking at invisible files within the package would also do the trick (i.e. use Onyx to make all files visible).