In order for you to use Site Studio on your existing website, you need to add our cohesion-placeholder tag to your custom theme.
Site Studio uses render placeholders of inline CSS and as these cannot be naturally injected into the HTML template, the site's theme needs to contain <cohesion-placeholder></cohesion-placeholder> within the <head> tag.
To enable Site Studio on a custom theme you must add cohesion: true to your themes .info.yml file.