XB block header renders incorrectly in site editor

[rmccue]

Steps to reproduce:

1. Open the (FSE) site editor in latest Altis (Appearance > Editor)
2. Add an XB (either will do)
3. Observe XB header is not styled correctly

This should be styled like the header in the (content) block editor, but instead is missing all styling.

Acceptance criteria:

• XB interface render correctly when using the FSE

[robindevitt]

Bringing this back to the backlog for the next sprint so there can be some more time to replicate and fix the issue.