I've done it
As gofer said, the easy bit was copying the TCP definitions (and their image sub-folders) from one theme to another. It then took several hours to modify my image files and sort out the image positioning. (And I think I know what I'm doing.
)
Oh and I renamed the Layouts so that my Layouts window mappings were still OK.
I'm not sure that I've finished yet. I'm using the combined theme here and fix it when I spot a glitch.