Teach Sky buttons and dialogs how to use the new Theme system

sky/sdk/lib/widgets/dialog.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 '../theme/colors.dart' as colors;
 import 'basic.dart';
 import 'material.dart';
+import "theme.dart";
 
 class Dialog extends Component {
   Dialog({
     String key,
     this.title,
     this.content,
     this.actions,
     this.onDismiss
   }) : super(key: key);
 
   final Widget title;
   final Widget content;
   final Widget actions;
   final Function onDismiss;
 
+  Color get color {
+    switch (Theme.of(this).brightness) {
+      case ThemeBrightness.light:
+        return colors.White;
+      case ThemeBrightness.dark:
+        return colors.Grey[800];
+    }
+  }
+
   Widget build() {
     Container mask = new Container(
       decoration: const BoxDecoration(
         backgroundColor: const Color(0x7F000000)));
 
     List<Widget> children = new List<Widget>();
 
     if (title != null)
       children.add(title);
 
     if (content != null)
       children.add(content);
 
     if (actions != null)
       children.add(actions);
 
     return new Stack([
       new Listener(
         child: mask,
         onGestureTap: (_) => onDismiss()
       ),
       new Center(
         child: new ConstrainedBox(
           constraints: new BoxConstraints(minWidth: 280.0),
           child: new Material(
             level: 4,
+            color: color,
             child: new ShrinkWrapWidth(
               child: new Block(children)
             )
           )
         )
       )
     ]);
   }
 }