Out of interest, are they automatically classed as light or dark? For instance by some heuristic or algorithm. I saw it's reporting some contrast measures which is handy.
I wonder how feasible it might be to use some colour distance heuristics to find similar themes: then if you find one you mostly like but it's slightly off due to some preference, you can still easily browse that "neighborhood"
They're classified automatically, though I did make it so I can change the light/dark classification in the Admin tools if something is off.
Since the Similar Themes functionality was built, there has been work done on accessibility, tags, and the color profiles, so Similar may need to be revisited for its classification. I'll note to check that
Also, there is a Find My Theme link in the nav that asks a few visual preference questions, then shows themes related to the answers. If you have feedback about that, and what works/doesn't work well for you, I'd love to hear it!
Thanks!