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

sky/examples/stocks2/lib/stock_home.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/editing/input.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/drawer.dart';
 import 'package:sky/widgets/drawer_header.dart';
@@ skipping 151 matching lines @@
           icon: 'navigation/menu_white',
           onPressed: _handleOpenDrawer),
         center: new Text('Stocks', style: typography.white.title),
         right: [
           new IconButton(
             icon: 'action/search_white',
             onPressed: _handleSearchBegin),
           new IconButton(
             icon: 'navigation/more_vert_white',
             onPressed: _handleMenuShow)
-        ],
-        backgroundColor: colors.Purple[500]
+        ]
       );
   }
 
   // TODO(abarth): Should we factor this into a SearchBar in the framework?
   Widget buildSearchBar() {
     return new ToolBar(
       left: new IconButton(
         icon: 'navigation/arrow_back_grey600',
         onPressed: _handleSearchEnd),
       center: new Input(
@@ skipping 24 matching lines @@
         floatingActionButton: new FloatingActionButton(
           child: new Icon(type: 'content/add_white', size: 24)
         ),
         drawer: _drawerShowing ? buildDrawer() : null
       ),
     ];
     addMenuToOverlays(overlays);
     return new Stack(overlays);
   }
 }