Cupertino dark theme

main.dart

import 'package:flutter/cupertino.dart';

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

class TestApp extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    return const CupertinoApp(
        home: CupertinoTheme(
            data: CupertinoThemeData(brightness: Brightness.dark),
            child: CupertinoPageScaffold(
              navigationBar: CupertinoNavigationBar(middle: Text("Test")),
              child: Center(child: Text('Hello world!')),
            )));
  }
}