How to do it...

Open your IDE and run the app. Let's start theming the e-commerce screen by adding a theme to your MaterialApp widget:

1. Open main.dart and add the following code:

class StaticApp extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    return MaterialApp(
      theme: ThemeData(
        brightness: Brightness.dark,
        primaryColor: Colors.green,
      ),
      home: ECommerceScreen(),
    );
  }
}

2. Now that the Theme has been declared, the rest of the recipe will about deleting code so that the theme traverses down to the appropriate widgets.  
3. In the Scaffold of the EcommerceScreen class, delete the property and value for backgroundColor. In the _buildAppBar method, also delete these two lines:

backgroundColor: Colors.purpleAccent,
elevation: 0, 

When you hot reload, the AppBar will be green, respecting the primaryColor property of the app's theme.

4. The toggle bar could use a bit...