ScaffoldMessenger theming issue

main.dart

import 'package:flutter/material.dart';

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

final _scaffoldMessengerKey = GlobalKey<ScaffoldMessengerState>();

void showThemedSnackBar() {
  final messenger = _scaffoldMessengerKey.currentState!;
  final context = _scaffoldMessengerKey.currentContext!;

  // Theme is not the `MaterialApp.theme` as below because
  // it is not an ancestor of the default ScaffoldMessenger
  final theme = Theme.of(context);
  final snackBar = SnackBar(
    content: Text(
      'Themed message',
      style: TextStyle(
        color: theme.colorScheme.onPrimary,
      ),
    ),
    backgroundColor: theme.colorScheme.primary,
  );
  messenger.showSnackBar(snackBar);
}

class MyApp extends StatelessWidget {
  const MyApp({super.key});

  @override
  Widget build(BuildContext context) {
    return MaterialApp(
      // Remove scaffoldMessengerKey below for the workaround
      scaffoldMessengerKey: _scaffoldMessengerKey,
      theme: ThemeData(
        colorScheme: ColorScheme.light(
          primary: Colors.red,
          onPrimary: Colors.yellow,
        ),
      ),
      home: Scaffold(
        body: Center(
          child: ElevatedButton(
            onPressed: showThemedSnackBar,
            child: Text('Show snackbar'),
          ),
        ),
      ),
      // Workaround (remove the MaterialApp.scaffoldMessengerKey above)
      // builder: (_, child) => ScaffoldMessenger(
      //   key: _scaffoldMessengerKey,
      //   child: child!,
      // ),
    );
  }
}