What problem does this address?

Users should be able to select style variations from any theme - not just the ones that ship with their theme.

What is your proposed solution?

There are a few options:

1. A styles gallery of all styles for all themes, so users can choose the one they want - like the pattern gallery
2. We allow the site editor to access styles for all installed themes so users have a much bigger choice.

This work will probably depend on getting this issue closed: #42732

We also should consider how users could preview their theme with these styles in a gallery of tiles, rather than having to click and select each one.