I like the idea a lot. We actually already distribute the themes not only with the macOS builds but also with the Windows builds.
I wasn't aware of the licensing issue. I would say let's include those themes where we are sure about license compatibility and then try to relicense more and more themes either under MIT or BSD or GPLv2, so they are compatible. The relicensing process probably will take time and get tedious as previous authors won't respond. Still, I think it's worth a try.