custom_theme.dart

import 'package:flutter/material.dart';

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

class MyApp extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    return MaterialApp(
      title: 'Material App',
      builder: (context, child) {
        if (child == null) return const SizedBox.shrink();
        final brightness = Theme.of(context).brightness;
        IsmailThemeData theme;
        if (brightness == Brightness.dark) {
          theme = IsmailThemeData.dark();
        } else {
          theme = IsmailThemeData.light();
        }
        return IsmailTheme(
          data: theme,
          child: child,
        );
      },
      home: Home(),
    );
  }
}

class Home extends StatelessWidget {
  const Home({
    Key? key,
  }) : super(key: key);

  @override
  Widget build(BuildContext context) {
    return Scaffold(
      backgroundColor: IsmailTheme.of(context).backgroundColor,
      appBar: AppBar(
        title: Text('Material App Bar'),
      ),
      body: Center(
        child: Container(
          child: Text('Hello World'),
        ),
      ),
    );
  }
}

class IsmailThemeData {
  final Color backgroundColor;

  IsmailThemeData({required this.backgroundColor});
  factory IsmailThemeData.dark() =>
      IsmailThemeData(backgroundColor: Colors.green);
  factory IsmailThemeData.light() =>
      IsmailThemeData(backgroundColor: Colors.blue);

  @override
  bool operator ==(Object other) {
    if (identical(this, other)) return true;
  
    return other is IsmailThemeData &&
      other.backgroundColor == backgroundColor;
  }

  @override
  int get hashCode => backgroundColor.hashCode;

  IsmailThemeData copyWith({
    Color? backgroundColor,
  }) {
    return IsmailThemeData(
      backgroundColor: backgroundColor ?? this.backgroundColor,
    );
  }
}

class IsmailTheme extends StatelessWidget {
  const IsmailTheme({
    Key? key,
    required this.child,
    required this.data,
  }) : super(key: key);
  final IsmailThemeData data;
  final Widget child;

  static IsmailThemeData of(BuildContext context) =>
      context.dependOnInheritedWidgetOfExactType<_IsmailThemeScope>()!.theme;

  @override
  Widget build(BuildContext context) {
    return _IsmailThemeScope(
      theme: data,
      child: child,
    );
  }
}

class _IsmailThemeScope extends InheritedWidget {
  const _IsmailThemeScope({
    Key? key,
    required Widget child,
    required this.theme,
  }) : super(key: key, child: child);
  final IsmailThemeData theme;

  @override
  bool updateShouldNotify(_IsmailThemeScope oldWidget) =>
      theme != oldWidget.theme;
}