Block Theme Previews: simplify Site View text or consider splitting out theme name

[annezazu]

This was raised by @carolinan as part of the FSE Outreach Program's Rapid Revamp call for testing:

In the Navigation Sidebar, at the top, when the theme name is short, it looks like the theme name contains the word Design: “Previewing Atlas: Design”. While the longer combinations like “Previewing Atlas:
Templates” that span over two rows looked very tightly squeezed in between the Back button and the “Add new template” button.

Current view:

I think we could either split out the "preview" text or find a way to emphasize the text differently and make it fix the space better. I'll leave it to @WordPress/gutenberg-design for more thoughts!

[jasmussen]

It could just say "Previewing", and the theme name could have a unit of its own in the sidebar, potentially. It needs a design, but the context for what you're doing is pretty important and might need a "notice" style piece.

[jameskoster]

I'd vote for a dedicated element (such as the notice suggestion) rather than prefixing the panel title which feels a bit cumbersome.

It can potentially be combined with the Activate button?