Describe relationship to theme-ui theme specification

[jaronheard]

Currently, it is difficult to tell what the relationship, if any, between this theme specification and the theme-ui theme specification.

On system-ui.com, it says:

It is also the home for the Theme UI and DesignQL libraries that are built upon the foundation of this specification.

However, neither theme page describes the relationship between them. My best guess after examining both pages, without being a current user of either library, is as follows:

The theme specification for theme-ui is an extension of the system-ui theme specification with additional properties

The system-ui theme specification is used as the foundation for the theme-ui theme specification, which adds additional properties

If I'm understanding this correctly, I'd be willing to make a PR 💖