customization_appbar.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(
        appBar: MyAppBar(title: Text('MyAppBar')),
        body: Center(
          child: MyWidget(),
        ),
      ),
    );
  }
}

class MyWidget extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    return Text('Hello, World!', style: Theme.of(context).textTheme.headline4);
  }
}

class MyAppBar extends AppBar implements PreferredSizeWidget {
  MyAppBar({
    key,
    leading,
    automaticallyImplyLeading = true,
    title,
    actions,
    flexibleSpace,
    bottom,
    elevation,
    shape,
    backgroundColor,
    brightness,
    iconTheme,
    actionsIconTheme,
    textTheme,
    primary = true,
    centerTitle,
    excludeHeaderSemantics = false,
    titleSpacing = NavigationToolbar.kMiddleSpacing,
    toolbarOpacity = 1.0,
    bottomOpacity = 1.0,
  }) : super(
          key: key,
          leading: leading,
          automaticallyImplyLeading: automaticallyImplyLeading,
          title: title,
          actions: actions,
          flexibleSpace: flexibleSpace,
          bottom: bottom,
          elevation: elevation,
          shape: shape,
          backgroundColor: backgroundColor,
          brightness: brightness,
          iconTheme: iconTheme,
          actionsIconTheme: actionsIconTheme,
          textTheme: textTheme,
          primary: primary,
          centerTitle: centerTitle,
          excludeHeaderSemantics: excludeHeaderSemantics,
          titleSpacing: titleSpacing,
          toolbarOpacity: toolbarOpacity,
          bottomOpacity: bottomOpacity,
        );
  @override
  Size get preferredSize => Size.fromHeight(24);
}