Material dark mode

main.dart

import 'package:flutter/material.dart';

void main() => runApp(TestApp());

class TestApp extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    return MaterialApp(
        home: Theme(
            data: ThemeData(brightness: Brightness.dark),
            child: Scaffold(
                appBar: AppBar(
                  title: const Text('Test'),
                ),
                body: const Center(
                  child: Column(children: [Text('Hello world!')]),
                ))));
  }
}