Material light and dark themes for Sky widgets

sky/examples/stocks2/lib/stock_row.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/rendering/box.dart';
-import 'package:sky/theme/typography.dart' as typography;
 import 'package:sky/widgets/ink_well.dart';
 import 'package:sky/widgets/basic.dart';
+import 'package:sky/widgets/theme.dart';
 
 import 'stock_arrow.dart';
 import 'stock_data.dart';
 
 class StockRow extends Component {
 
   StockRow({ Stock stock }) : this.stock = stock, super(key: stock.symbol);
 
   final Stock stock;
 
@@ skipping 16 matching lines @@
       ),
       new Flexible(
         child: new Text(
           lastSale,
           style: const TextStyle(textAlign: TextAlign.right)
         )
       ),
       new Flexible(
         child: new Text(
           changeInPrice,
-          style: typography.black.caption.copyWith(textAlign: TextAlign.right)
+          style: Theme.of(this).text.caption.copyWith(textAlign: TextAlign.right)
         )
       )
     ];
 
     // TODO(hansmuller): An explicit |height| shouldn't be needed
     return new InkWell(
       child: new Container(
         padding: const EdgeDims(16.0, 16.0, 20.0, 16.0),
         height: kHeight,
         decoration: const BoxDecoration(
             border: const Border(
                 bottom: const BorderSide(color: const Color(0xFFF4F4F4)))),
         child: new Flex(children)
       )
     );
   }
 }