Description

If you have a non-FSE theme active, go to the Customizer, click the "Change" button next to the name of the active theme, and select a FSE theme to preview, the preview that gets loaded doesn't include any of the theme.json styles in the theme.

The preview does work if you have a FSE theme active (like Blockbase) and preview a different FSE theme (like TT1 Blocks), but not if your active theme is a non-FSE theme.

Step-by-step reproduction instructions

1. Activate a non-FSE theme, like Twenty Twenty-One.
2. Go to the Customizer.
3. Next to the theme name, click the "Change" button and select a FSE theme, like TT1 Blocks.
4. The preview is loaded, but the theme.json styles are not output, resulting in the theme looking broken.

Screenshots, screen recording, code snippet

Environment info

WordPress 5.8, Gutenberg 11.4.0

Please confirm that you have searched existing issues in the repo.

Yes

Please confirm that you have tested with all plugins deactivated except Gutenberg.

Yes