mobile-app/lib/app/my_app.dart

import 'package:farm_tpf/app/themes.dart';
import 'package:farm_tpf/utils/const_string.dart';
import 'package:flutter/material.dart';

class MyApp extends StatelessWidget {
  // This widget is the root of your application.
  @override
  Widget build(BuildContext context) {
    return MaterialApp(
      title: 'Flutter Demo',
      theme: AppTheme.of(context, listen: true).currentTheme,
      home: MyHomePage(title: 'Flutter Demo Home Page'),
    );
  }
}

class MyHomePage extends StatefulWidget {
  MyHomePage({Key key, this.title}) : super(key: key);

  final String title;

  @override
  _MyHomePageState createState() => _MyHomePageState();
}

class _MyHomePageState extends State<MyHomePage> {
  int _counter = 0;

  AppTheme _theme;

  @override
  void didChangeDependencies() {
    if (_theme == null) {
      _theme = AppTheme.of(context);
    }

    super.didChangeDependencies();
  }

  void _incrementCounter() {
    setState(() {
      // This call to setState tells the Flutter framework that something has
      // changed in this State, which causes it to rerun the build method below
      // so that the display can reflect the updated values. If we changed
      // _counter without calling setState(), then the build method would not be
      // called again, and so nothing would appear to happen.
      _counter++;
    });
  }

  @override
  Widget build(BuildContext context) {
    return Scaffold(
        appBar: AppBar(
          title: Text(widget.title),
          actions: [
            IconButton(
              icon: Icon(Icons.flip, color: Colors.white),
              onPressed: _theme?.switchTheme,
            ),
          ],
        ),
        body: Center(child: Text(app_name)));
  }
}