There are some others whom are already providing FREE or monetizing Pinegrow with Blocks, Plugins, etc.,
For Example:
I also thought I saw @matjaz mention that the API or Plugins would be developed further in Pinegrow 3, or that at least the Documentation in these areas would be improved? But I can’t seem to find the reference I was thinking of here or on Slack, so perhaps I was just dreaming of that or thinking of another app?
For an example of a Plugin you can reference the Pinegrow Atom Plugin.
Unless by “Themes” you meant changing the actual Pinegrow UI (colors/fonts/etc)? Currently those settings are in the app bundle and not as easily accessible or readily exposed for easy manipulation or sharing. You can however see some references for changing the look and feel of the Pinegrow UI in this thread about a Light Theme. Which I also thought there was mention of a Light Theme coming in Pinegrow 3?
Hope something proves useful to your inquiry.
What did you have in mind ?