Just as a tip/warning. Themes tend to fall behind unless maintained and it’s a fair bit of work just curating a list of currently working themes. A means of rating working themes or tagging them with a release is probably a good feature. A repo containing a mix of broken and working themes is annoying unless clearly advertised as such.
edit: A more ad hoc approach based on a discuss thread could have a sticky with working themes. The thread could of course point to said repo. This is just to signal the ad hoc ness and have easy feedback.