improve divider appearance in dark mode

sky/sdk/example/stocks/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/widgets/ink_well.dart';
 import 'package:sky/widgets/basic.dart';
 import 'package:sky/widgets/theme.dart';
 
@@ skipping 35 matching lines @@
           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))
+        decoration: new BoxDecoration(
+          border: new Border(
+            bottom: new BorderSide(color: Theme.of(this).dividerColor)
           )
         ),
         child: new Flex(children)
       )
     );
   }
 }