Core WordPress has a feature in the Customizer to preview different themes on the content directly within the Customizer. This is disabled on WP.com, I think... because of performance? Let's consider revisiting that decision; maybe those concerns are no longer relevant, or our focus on promoting certain kinds of forward-looking themes means we could limit the available themes to a handful.
This could have been performance-related (loading 300+ themes at the time). If we want to focus on our "recommended themes", then loading twenty or so themes should be doable.
Core WordPress has a feature in the Customizer to preview different themes on the content directly within the Customizer. This is disabled on WP.com, I think... because of performance? Let's consider revisiting that decision; maybe those concerns are no longer relevant, or our focus on promoting certain kinds of forward-looking themes means we could limit the available themes to a handful.