Remove Material's opinion about background colours, since in fact the background colour varies dram…

sky/sdk/example/widgets/tabs.dart

 // Copyright 2015 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 import 'package:sky/painting/text_style.dart';
-import 'package:sky/theme/typography.dart';
+import 'package:sky/theme/colors.dart' as colors;
+import 'package:sky/theme/typography.dart' as typography;
 import 'package:sky/widgets/basic.dart';
 import 'package:sky/widgets/material.dart';
 import 'package:sky/widgets/scaffold.dart';
 import 'package:sky/widgets/tabs.dart';
 import 'package:sky/widgets/tool_bar.dart';
 import 'package:sky/widgets/widget.dart';
 
 class TabbedNavigatorApp extends App {
   int selectedIndex = 0;
 
@@ skipping 21 matching lines @@
 
     TabNavigator tabNavigator = new TabNavigator(
       views: views,
       selectedIndex: selectedIndex,
       onChanged: (tabIndex) {
         setState(() { selectedIndex = tabIndex; } );
       }
     );
 
     ToolBar toolbar = new ToolBar(
-      center: new Text('Tabbed Navigator', style: white.title)
+      center: new Text('Tabbed Navigator', style: typography.white.title)
     );
 
     return new Scaffold(
       toolbar: toolbar,
-      body: new Material(child: tabNavigator)
+      body: new Material(
+        color: colors.Grey[50],
+        child: tabNavigator
+      )
     );
   }
 }
 
 void main() {
   runApp(new TabbedNavigatorApp());
 }