Add a new Theme widget to control color and text color of apps

sky/sdk/lib/widgets/tool_bar.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/widgets/theme.dart';
+
 import '../rendering/flex.dart';
 import '../theme/shadows.dart';
 import '../theme/view_configuration.dart';
 import 'basic.dart';
 
 class ToolBar extends Component {
 
   ToolBar({
     String key,
     this.left,
@@ skipping 26 matching lines @@
     if (right != null)
       children.addAll(right);
 
     return new Container(
       child: new Flex(
         [new Container(child: new Flex(children), height: kToolBarHeight)],
         alignItems: FlexAlignItems.flexEnd
       ),
       padding: new EdgeDims.symmetric(horizontal: 8.0),
       decoration: new BoxDecoration(
-        backgroundColor: backgroundColor,
+        backgroundColor: backgroundColor == null ? Theme.of(this).color[500] : backgroundColor,
         boxShadow: shadows[2]
       )
     );
   }
 
 }