I was emptying the trash and noticed some images in a "recovered files" directory named something like
sotryboard-paper-green-flat.tiff with variations for colour, gradients, etc. - These seemed to be variations of the default background and shape-fill images for the Keynote "Storyboard" theme, I had recently been making a Keynote using that theme so I figured it was some temporary files relating to that. "Great!" I thought, "I'll go and check out how to use those variations later."
So now back in Keynote using the Storyboard theme, when I create shapes they have a blue (tiled) fill-image and if I select Choose... it takes me to my own
PresentationAssets directory from where I've loaded up images before. I don't know where to look for these built-in textures which are designed to fit with the theme, how do I find and use these images?