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

sky/sdk/lib/widgets/theme.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 'package:sky/theme/theme_data.dart';
+import 'basic.dart';
+import 'widget.dart';
+
+class Theme extends Inherited {
+
+  Theme({
+    String key,
+    this.data,
+    Widget child
+  }) : super(key: key, child: child) {
+    assert(child != null);
+    assert(data != null);
+  }
+
+  final ThemeData data;
+
+  static ThemeData of(Component component) {
+    Theme theme = component.inheritedOfType(Theme);
+    // If you hit this assert, you need to wrap your Component in a Theme
+    assert(theme != null);
+    return theme.data;
+  }
+}