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

sky/sdk/lib/widgets/checkbox.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/colors.dart' as colors;
+import 'package:sky/widgets/theme.dart';
 
 import 'basic.dart';
 import 'toggleable.dart';
 export 'toggleable.dart' show ValueChanged;
 
 const double _kMidpoint = 0.5;
 const sky.Color _kUncheckedColor = const sky.Color(0x8A000000);
-// TODO(jackson): This should change colors with the theme
-sky.Color _kCheckedColor = colors.Purple[500];
 const double _kEdgeSize = 20.0;
 const double _kEdgeRadius = 1.0;
 
 class Checkbox extends Toggleable {
 
   Checkbox({
     String key,
     bool value,
     ValueChanged onChanged
   }) : super(key: key, value: value, onChanged: onChanged);
@@ skipping 25 matching lines @@
         )
       );
       canvas.drawRRect(rrect, paint);
     }
 
     if (toggleAnimation.value > _kMidpoint) {
       double t = (toggleAnimation.value - _kMidpoint) / (1.0 - _kMidpoint);
 
       // Solid filled rrect
       paint.setStyle(sky.PaintingStyle.strokeAndFill);
+      Color themeColor = Theme.of(this).color[500];
       paint.color = new Color.fromARGB((t * 255).floor(),
-                                       _kCheckedColor.red,
-                                       _kCheckedColor.green,
-                                       _kCheckedColor.blue);
+                                       themeColor.red,
+                                       themeColor.green,
+                                       themeColor.blue);
       canvas.drawRRect(rrect, paint);
 
       // White inner check
       paint.color = const sky.Color(0xFFFFFFFF);
       paint.setStyle(sky.PaintingStyle.stroke);
       sky.Path path = new sky.Path();
       sky.Point start = new sky.Point(_kEdgeSize * 0.2, _kEdgeSize * 0.5);
       sky.Point mid = new sky.Point(_kEdgeSize * 0.4, _kEdgeSize * 0.7);
       sky.Point end = new sky.Point(_kEdgeSize * 0.8, _kEdgeSize * 0.3);
       Point lerp(Point p1, Point p2, double t)
         => new Point(p1.x * (1.0 - t) + p2.x * t, p1.y * (1.0 - t) + p2.y * t);
       sky.Point drawStart = lerp(start, mid, 1.0 - t);
       sky.Point drawEnd = lerp(mid, end, t);
       path.moveTo(drawStart.x, drawStart.y);
       path.lineTo(mid.x, mid.y);
       path.lineTo(drawEnd.x, drawEnd.y);
       canvas.drawPath(path, paint);
     }
   }
 }