fix(theme): apply base theme classes at location the theme is included (#10737)

Author: josephperrott

src/lib/core/_core.scss

@@ -39,7 +39,8 @@
 
   // Wrapper element that provides the theme background when the
   // user's content isn't inside of a `mat-sidenav-container`.
-  .mat-app-background {
+  .mat-app-background,
+  &.mat-app-background {
     $background: map-get($theme, background);
     $foreground: map-get($theme, foreground);
 
@@ -48,7 +49,9 @@
   }
 
   // Marker that is used to determine whether the user has added a theme to their page.
-  .mat-theme-loaded-marker {
-    display: none;
+  @at-root {
+    .mat-theme-loaded-marker {
+      display: none;
+    }
   }
 }