Explore creation of block based themes

[gusaus]

From the start, we've included and update all the themes that ship with Newspack (https://github.com/Automattic/newspack-theme) + a custom child theme (https://github.com/OpenProducer/newspack-platform/tree/master/wp-content/themes/newspack-theme-child).

Problem is that these types of 'classic' themes have been gradually replaced with block themes in WordPress world (see https://www.youtube.com/watch?v=dZ1kruAnQso for a quick comparison).

While it's understandable why Newspack wouldn't want to include something that would be easier for end users to break, it looks like they're now actively working on a block theme https://github.com/Automattic/newspack-block-theme.

Assuming their goal is to make it the default theme during install, it might be worth testing and exploring whether or not we can leverage to create themes for specific branches (such as https://github.com/OpenProducer/newspack-platform/tree/radio).