[fieldgroup] CSS reaches into child component.

[Westbrook]

https://github.com/adobe/spectrum-css/blob/92c045f540f2ab3578512abd76c94eda9c6faad6/components/fieldgroup/index.css#L24-L38

The above code reaches into the Checkbox component to apply styles rather the interacting with a CSS Custom Property API surfaced on that component making it difficult to apply in a portable way when pairing this pattern with shadow DOM in SWC.