Show ready theme in the Preview mode

The preview is generated via the Playground, which you as the theme developer can control via blueprint.json. See: https://developer.www.ads-software.com/plugins/wordpress-org/previews-and-blueprints/

Some examples (also of plugins) can be found here: https://github.com/WordPress/blueprints

I have added to my plugins via blueprint.js, for example, that individual data records are created when the preview is loaded. This provides the Playground environment with the additional data that can make a preview look more appropriate. I think this way you can also set the settings that are otherwise made manually in the Customizer.