fix(material-experimental/mdc-core): add app background color (#22992)
fix(material-experimental/mdc-core): add app background color (#22992)

Adds a `mat-mdc-app-background` utility class similar to `mat-app-background`.
angular/components,crisbeto/material2,angular/components,crisbeto/material2,crisbeto/material2,angular/components,angular/components,crisbeto/material2