As far as I can tell, there are no additional “themes” out there. But I did find the SVG files the clock uses in
It would be easy enough to replace these; ideally I guess there would be a section in the GNOME Do wiki to upload clock theme packages. The hands of the clock are another matter, generated by Do itself. As it is, I’ve never created an SVG in my life, so I’m stuck with a numberless clock for now.