flutter 怎么实现app整体灰色效果

class MyApp extends StatelessWidget {
 // This widget is the root of your application.
 @override
 Widget build(BuildContext context) {
  return ShaderMask(
   child: OKToast(
    child: MaterialApp(
     title: 'Pick Image Demo',
     theme: ThemeData(
      primarySwatch: Colors.lime,
     ),
     home: MyHomePage(title: 'Pick Image Demo'),
    ),
   ),
   shaderCallback: (Rect bounds) {
    return ui.Gradient.linear(Offset.zero,
      Offset(bounds.width, bounds.height), [Colors.grey, Colors.grey]);
   },
   // blendMode: BlendMode.dst,
   blendMode: BlendMode.saturation,
  );
 }
}