Banner outer elevation and radius are not theme-targetable #6006

Problem

Banner's documented banner theme target is attached to the coloured header. The outer element separately owns the elevation shadow and the outer radius that shapes that shadow for elevated card banners, but it has no stable theme target.

A theme that needs a different elevation treatment or outer radius therefore has to depend on Banner's DOM structure rather than a documented API.

Expected behaviour

Themes should have a stable public way to style the outer shadow and radius across card and section containers and every elevation value. Existing Banner targets and default rendering should remain unchanged.

Context

#5483 established the component's current sub-target naming and documentation pattern.