Continuing the discussion from Successful Google Summer of Code 2016 Projects:
the Plone.org/download page now has a section on themes. Yes it's a (slow) start, but it's a start. There is a folder in there (currently excluded from nav) for themes, for when we are ready to do something more significant and less manual perhaps. Then we can have the theming control panel include a link to (for example) Plone.org/download/themes or somesuch.