While I have been supporting and maintaining WordPress since 2014 and becoming a Theme Reviewer for the WordPress Theme Team. I am for sure I can say is that I now support ClassicPress
I have fully intended to create a ClassicPress theme when ClassicPress was first introduced but I wanted to wait for a while to see how far ClassicPress can go and now I can now introduced you a theme Amicable, a theme that works with ClassicPress. Amicable is actually a clone of my latest WordPress theme, Individuality. The only slight difference Gutenberg Block Editor is no where be found in Amicable theme.
As Amicable continues to grow in features in later updates. Creating themes for ClassicPress will continue to grow as ClassicPress continues to be there.