fix(sidenav): apply theming on sidenav correctly

- The current theming of the sidenav wasn't working, because theming wasn't initialized on the element.
- Backdrop should only inherit the theme if there is a parent (just a performance improvement - remove end-less non-sense watcher)

Fixes #7084

  Closes #7374

Author: devversion

src/components/backdrop/backdrop.js

@@ -60,7 +60,10 @@ angular
           }
         }
 
-        $mdTheming.inherit(element, element.parent());
+        // Only inherit the parent if the backdrop has a parent.
+        if (element.parent().length) {
+          $mdTheming.inherit(element, element.parent());
+        }
       });
 
     }

src/components/sidenav/sidenav.js

@@ -242,6 +242,10 @@ function SidenavDirective($mdMedia, $mdUtil, $mdConstant, $mdTheming, $animate,
     };
     var backdrop = $mdUtil.createBackdrop(scope, "md-sidenav-backdrop md-opaque ng-enter");
 
+    $mdTheming(element);
+
+    // The backdrop should inherit the sidenavs theme,
+    // because the backdrop will take its parent theme by default.
     $mdTheming.inherit(backdrop, element);
 
     element.on('$destroy', function() {