fullscreen demo

main.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(),
        ),
      ),
    );
  }
}

class MyWidget extends StatelessWidget {

  // Copy code from https://mrflutter.com/fullscreen-page-with-transparent-appbar-in-flutter/
  
  @override
  Widget build(BuildContext context) {
    return Scaffold(
      body: Stack(
        children: <Widget>[
          Container(
            decoration: BoxDecoration(
              image: DecorationImage(
                image: NetworkImage(
                    'https://images.unsplash.com/photo-1517030330234-94c4fb948ebc?ixlib=rb-1.2.1&ixid=eyJhcHBfaWQiOjEyMDd9&auto=format&fit=crop&w=1275&q=80'),
                fit: BoxFit.cover,
              ),
            ),
            child: Center(
              child: Text(
                'mrflutter.com',
                style: TextStyle(
                  color: Colors.white,
                  fontSize: 30,
                ),
              ),
            ),
          ),
          Positioned(
            child: AppBar(
              title: Text("Transparent AppBar"),
              backgroundColor: Colors.transparent,
              elevation: 0,
              actions: <Widget>[
                IconButton(
                  icon: Icon(Icons.share),
                  onPressed: () {},
                  tooltip: 'Share',
                ),
              ],
            ),
          )
        ],
      ),
    );
  }
}