Additional CSS option is unavailable on FSE themes on Starter Plans with Custom CSS subscription

[staproehl]

Steps to replicate

1. Add Custom CSS subscription to Starter Plan
2. Install FSE theme (original report came from Assembler, but was tested on Twenty Twenty Four as well)
3. Go to Appearance menu item (See first result)
4. Go to Editor -> Styles -> Sidebar Style panel -> More icon with three dots (See second result)

Browsers: Firefox and Chrome

Result

First result - Additional CSS is not present in Customizer.

Second result - Additional CSS is not present in Site Editor.

Expected

First result - Even though its an FSE theme, sometimes Customizer is available so the expected outcome isn't definitive and depends on if Customizer should be available with FSE themes. In these testings and the original user report, Additional CSS in Customizer was not available. However, HE SU'd into user's site and could access Customizer, which showed custom CSS added, even though the user reported not having access to it. These are screenshots of the SU with the assembler theme, where HE could see option for Additional CSS in Customizer but not Site Editor:

Second result - It is expected that the user has access to Additional CSS from the More icon in Site Editor, just as described in this guide: https://wordpress.com/support/editing-css/#access-the-css-editor-via-styles.

Notes: Additional CSS via Customizer is available with a Classic theme.

• User report: [8365815-zen]

[github-actions[bot]]

Support References

This comment is automatically generated. Please do not edit it.

• [ ] 8365815-zen

[staproehl]

I realized I should have reported this in Calypso, not in themes so I made a new report here: https://github.com/Automattic/wp-calypso/issues/92014

[staproehl]

Closing as this is the incorrect place to report it.