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

sky/examples/stocks2/lib/stock_app.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/rendering/sky_binding.dart';
+import 'package:sky/theme/colors.dart' as colors;
+import 'package:sky/theme/theme_data.dart';
+import 'package:sky/theme/typography.dart' as typography;
 import 'package:sky/widgets/basic.dart';
 import 'package:sky/widgets/navigator.dart';
+import 'package:sky/widgets/theme.dart';
 import 'package:sky/widgets/widget.dart';
 
 import 'stock_data.dart';
 import 'stock_home.dart';
 import 'stock_settings.dart';
 
 class StocksApp extends App {
 
   StocksApp() {
     _navigationState = new NavigationState([
@@ skipping 21 matching lines @@
   NavigationState _navigationState;
 
   void onBack() {
     setState(() {
       _navigationState.pop();
     });
     // TODO(jackson): Need a way to invoke default back behavior here
   }
 
   Widget build() {
-    return new Navigator(_navigationState);
+    return new Theme(
+      data: new ThemeData(color: colors.Purple, text: typography.white),
+      child: new Navigator(_navigationState)
+    );
   }
 }
 
 void main() {
   print("starting stocks app!");
   runApp(new StocksApp());
   SkyBinding.instance.onFrame = () {
     // uncomment this for debugging:
     // SkyBinding.instance.debugDumpRenderTree();
   };
 }