Refactoring to support dark theme better

sky/sdk/lib/widgets/tabs.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 'dart:math' as math;
 
 import 'package:sky/rendering/box.dart';
 import 'package:sky/rendering/object.dart';
 import 'package:sky/widgets/basic.dart';
 import 'package:sky/widgets/icon.dart';
@@ skipping 277 matching lines @@
     bool textAndIcons = false;
     int tabIndex = 0;
     for (TabLabel label in labels) {
       tabs.add(_toTab(label, tabIndex++));
       if (label.text != null && label.icon != null)
         textAndIcons = true;
     }
     return new TabBarWrapper(
       children: tabs, 
       selectedIndex: selectedIndex,
-      backgroundColor: Theme.of(this).primary[500],
-      indicatorColor: Theme.of(this).accent[200],
+      backgroundColor: Theme.of(this).primaryColor,
+      indicatorColor: Theme.of(this).accentColor,
       textAndIcons: textAndIcons
     );
   }
 }
 
 class TabNavigatorView {
   TabNavigatorView({ this.label, this.builder });
 
   final TabLabel label;
   final Builder builder;
@@ skipping 32 matching lines @@
       onChanged: _handleSelectedIndexChanged,
       selectedIndex: selectedIndex
     );
 
     Widget content = views[selectedIndex].buildContent();
     return new Flex([tabBar, new Flexible(child: content)],
       direction: FlexDirection.vertical
     );
   }
 }