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

sky/examples/stocks2/lib/stock_settings.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/theme/colors.dart' as colors;
-import 'package:sky/theme/typography.dart' as typography;
 import 'package:sky/widgets/basic.dart';
 import 'package:sky/widgets/checkbox.dart';
 import 'package:sky/widgets/icon_button.dart';
 import 'package:sky/widgets/menu_item.dart';
 import 'package:sky/widgets/navigator.dart';
 import 'package:sky/widgets/scaffold.dart';
+import 'package:sky/widgets/theme.dart';
 import 'package:sky/widgets/tool_bar.dart';
 
 class StockSettings extends Component {
 
   StockSettings(this._navigator);
 
   Navigator _navigator;
 
   bool _awesome = false;
   void _handleAwesomeChanged(bool value) {
     setState(() {
       _awesome = value;
     });
   }
 
   Widget buildToolBar() {
     return new ToolBar(
       left: new IconButton(
         icon: 'navigation/arrow_back_white',
         onPressed: _navigator.pop),
-      center: new Text('Settings', style: typography.white.title),
-      backgroundColor: colors.Purple[500]
+      center: new Text('Settings', style: Theme.of(this).text.title)
     );
   }
 
   Widget buildSettingsPane() {
     return new Container(
       padding: const EdgeDims.symmetric(vertical: 20.0),
       decoration: new BoxDecoration(backgroundColor: colors.Grey[50]),
       child: new Block([
         new MenuItem(
           icon: 'action/thumb_up',
           onPressed: () => _handleAwesomeChanged(!_awesome),
           children: [
             new Flexible(child: new Text('Everything is awesome')),
             new Checkbox(value: _awesome, onChanged: _handleAwesomeChanged)
           ]
         ),
       ])
     );
   }
 
   Widget build() {
     return new Scaffold(
       toolbar: buildToolBar(),
       body: buildSettingsPane()
     );
   }
 }