Here's the current experience loading the site editor:

A couple of issues to address:

• The FOWPA (Flash of wp-admin) is awkward. Can we hide it?
• The light grey background feels a bit broken. Can we use the dark grey background from the template editor, and isolated template part editor? Use a dark background for the site editor #35520
• The "Loading..." label in the top bar is easily missed. Something on the canvas would be more obvious.