A couple of questions:
1. How easy is it to customise and extend the interface of the editor while still using the core of Puck?
2. This looks like it could be used to build an editor similar to Shopify's theme editor or is that a stretch?
1. You can override a bunch of stuff using React-like APIs. `renderHeader` allows you to take over the header, and the plugin API allows you to wrap the root of the preview content page and take over the fields area on the right hand side. We don't yet support full take-overs, but we'll continue to work on these. Plugin API: https://github.com/measuredco/puck#plugins
2. I think you should be able to mimic a bunch of the behaviours. The main missing piece is multi-column support, which is landing soon via the `nested-dropzones` branch: https://github.com/measuredco/puck/pull/37