Yes a core part of what I’ve tried to do is separate content from presentation. This all started by trying to build an alt-protocol like Gemini or Gopher as a minimal writing and publishing experience. I realised that for it to be useful it needed to be accessible by browsers, so I added the SSG layer. But it still publishes the source content and config alongside the HTML. This means sites can be accessed by more than just browsers - as an experiment I made a CLI client that parses the source directly.
Themes can be imported via a URL or as a zip file and are pretty similar to Jekyll or 11ty themes. My next task is to port a bunch of them over. I’m also planning to create ready-configured template sites that users can copy and configure. Maybe take an Astro or Ghost approach, with a mix of free and paid themes.