Global Styles: Ability to manage partial style objects

[jameskoster]

In #40318 the section-specific theme.json concept is being explored. For example applying specific styles to a whole page, a template, or a group of blocks.

This idea sits somewhere between global style variations (which affect the entire site) and block style variations (which affect a single block). There's a mockup in #40318 (comment) which explains more visually.

Initially the UI may only articulate theme-bundled values, but eventually users should be afforded a way to create and manage their own style objects. In this issue we can explore designs for those management tasks.

[mtias]

This is going to become more and more important now we have style packages for blocks/patterns. The challenge would be to come up with an interface that doesn't feel overtly complicated for the most common use cases.