Following #3 (comment), I spent a few cycles iterating on this and came up with https://github.com/pradyunsg/sphinx-themes. I added a few themes to demonstrate the approach and a very basic/minimal index page for this, which is online now.

I started from-scratch and came away with quite a different approach for doing things. This resulted in:

• Generating demos, and screenshots (w/ selenium) in a GitHub Action job.
• Keep all theme-related information in a JSON file.
• No requirement on docker -- plain Python scripts w/ nox.
• No generated HTML/CSS/images are checked into the master branch.
• Preview images contain multiple viewport sizes! 🎉

Notably, the main changes due to all of this are:

• Significantly lower barrier for adding new themes! 🎉
• Themes have to be installable from PyPI, usable against the current Sphinx version
  • This is a reasonable assumption, and helps ensure that people can actually use the themes. :)
• No conf.py is included/provided
  • The theme should have documentation about this, so the gallery doesn't need to cover this.
  • It should be easy-enough to add this if we really want to (I don't)
• 2 column vs 3 column layout
  • I wanted this to include additional viewport sizes, so, yea, I think this design change is worthwhile. :)

This was mostly an over-the-boring-day thing, and there's definitely scope for improvement in the automation (eg: separating render-with-themes vs generate-screenshots). If there's interest in bringing in most (all?) of that codebase into this one, I'd like to hear about it! ^>^