What problem does this address?

Currently, only Style Variations support the title attribute not the default theme.json.

In the Global Styles panel in the Site Editor Default is hard-coded for naming the default.

What is your proposed solution?

It would be nice to prescribe a name other than Default.

• Perhaps the Theme Name (this wouldn't need to be automatic, as it may not be a desired experience in some Themes).
• Perhaps the default theme.json is Light that pairs nicely with a Style Variation named Dark.
• Perhaps a name disconnected from Theme Name entirely Whimsical Brand.

By supporting title in the top-level theme.json specification and using it to populate the UI, all those potential outcomes could be addressed.

Happy to open a PR to address.