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

Adds a `mat-mdc-app-background` utility class similar to `mat-app-background`.

Author: crisbeto

src/material-experimental/mdc-core/_core-theme.scss

@@ -1,3 +1,4 @@
+@use 'sass:map';
 @use '../../material/core/theming/theming';
 @use './option/option-theme';
 @use './option/optgroup-theme';
@@ -28,6 +29,17 @@
           @include elevation.private-theme-elevation($zValue, $color);
         }
       }
+
+      // Wrapper element that provides the theme background when the user's content isn't
+      // inside of a `mat-sidenav-container`. Note that we need to exclude the ampersand
+      // selector in case the mixin is included at the top level.
+      .mat-mdc-app-background#{if(&, ', &.mat-mdc-app-background', '')} {
+        $background: map.get($color, background);
+        $foreground: map.get($color, foreground);
+
+        background-color: theming.get-color-from-palette($background, background);
+        color: theming.get-color-from-palette($foreground, text);
+      }
     }
   }
 }