container.dart

import 'package:flutter/material.dart';

final Color darkBlue = Color.fromARGB(255, 18, 32, 47);

void main() {
  runApp(MyApp());
}

class MyApp extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    return MaterialApp(
      theme: ThemeData.dark().copyWith(scaffoldBackgroundColor: darkBlue),
      debugShowCheckedModeBanner: false,
      home: Scaffold(
        body: Center(
          child: MyWidget(color: Colors.white, size: 60),
        ),
      ),
    );
  }
}

class MyWidget extends Container {
  MyWidget({
    Color color,
    double size,
  }) : super(
          color: color,
          height: size,
          width: size,
        );
}