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

sky/sdk/lib/widgets/material.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 '../painting/box_painter.dart';
+import '../theme/colors.dart' as colors;
 import '../theme/edges.dart';
 import '../theme/shadows.dart';
 import 'basic.dart';
 import 'default_text_style.dart';
 import 'theme.dart';
 
 class Material extends Component {
 
   Material({
     String key,
     this.child,
     this.edge: MaterialEdge.card,
     this.level: 0,
     this.color
   }) : super(key: key);
 
   final Widget child;
   final int level;
   final MaterialEdge edge;
   final Color color;
 
+  Color get backgroundColor {
+    if (color != null)
+      return color;
+    switch (Theme.of(this).brightness) {
+      case ThemeBrightness.light:
+        return colors.Grey[50];
+      case ThemeBrightness.dark:
+        return colors.Grey[850];
+    }
+  }
+
   // TODO(ianh): we should make this animate level changes and color changes
 
   Widget build() {
     return new Container(
       decoration: new BoxDecoration(
         boxShadow: shadows[level],
         borderRadius: edges[edge],
-        backgroundColor: color == null ? Theme.of(this).backgroundColor : color,
+        backgroundColor: backgroundColor,
         shape: edge == MaterialEdge.circle ? Shape.circle : Shape.rectangle
       ),
       child: new DefaultTextStyle(style: Theme.of(this).text.body1, child: child)
     );
   }
 
 }