Hey @1080 - we don't currently offer a quick way to automatically enable theme extension app blocks. At the moment, we instead recommend that during the onboarding process for the app, you build in messaging that walks merchants through enabling it on their theme. There's a good overview here in our dev docs.
That said, we wanted to look into this a bit more for you and found a really good article from another Shopify developer that has some advice on how you can walk your merchants through enabling the app blocks - link to that here.
Hope this helps - let us know if we can clarify anything on our end.