[Stepper]: Selectors reach into classes/element that it does not own

[Westbrook]

Description

As seen in https://github.com/adobe/spectrum-css/blob/main/components/stepper/index.css#L216-L224 the CSS for Stepper reaches into the DOM/classes of In-field Button to customize it, rather than leveraging the --mod- APIs.

Steps to reproduce

1. Go to code

Expected behavior

Patterns should leverage the --mod- APIs surfaced by other patterns rather than reaching into DOM/classes that they do not own.

[pfulton]

Tracking this in Jira via CSS-630