A fever dream I had:
 
theme.profile { an appearance }, and we use that as the "hardcoded default" (or, I suppose the first layer).

That would let folks put the WHOLE appearance of the terminal into a theme, for sharing. background images, background colors, color schemes, fonts, you name it. Now it's just one blob you put into the settings.

Settings from dynamic profiles, profiles.defaults, and profile settings would all still take precedence.