Refactoring to support dark theme better

sky/sdk/lib/widgets/switch.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:sky' as sky;
 
 import 'package:sky/theme/shadows.dart';
 
 import '../painting/shadows.dart';
 import '../rendering/box.dart';
@@ skipping 19 matching lines @@
     bool value,
     ValueChanged onChanged
   }) : super(key: key, value: value, onChanged: onChanged);
 
   Size get size => const Size(_kSwitchWidth + 2.0, _kSwitchHeight + 2.0);
 
   void customPaintCallback(sky.Canvas canvas, Size size) {
     sky.Color thumbColor = _kThumbOffColor;
     sky.Color trackColor = _kTrackOffColor;
     if (value) {
-      thumbColor = Theme.of(this).primary[500];
+      thumbColor = Theme.of(this).accentColor;
       trackColor = new sky.Color(thumbColor.value & 0x80FFFFFF);
     }
 
     // Draw the track rrect
     sky.Paint paint = new sky.Paint()..color = trackColor;
     paint.setStyle(sky.PaintingStyle.fill);
     sky.Rect rect = new sky.Rect.fromLTRB(
       0.0,
       _kSwitchHeight / 2.0 - _kTrackHeight / 2.0,
       _kTrackWidth,
@@ skipping 11 matching lines @@
 
     // The thumb contracts slightly during the animation
     double inset = 2.0 - (toggleAnimation.value - 0.5).abs() * 2.0;
     Point thumbPos = new Point(
       _kTrackRadius + toggleAnimation.value * (_kTrackWidth - _kTrackRadius * 2),
       _kSwitchHeight / 2.0
     );
     canvas.drawCircle(thumbPos, _kThumbRadius - inset, paint);
   }
 }