Changes I made to get app to match what is shown in the Flutter theme lesson.

main.dart

import 'package:flutter/material.dart';

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

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

  @override
  Widget build(BuildContext context) {
    return MaterialApp(
      title: 'Flutter Demo',
      theme: ThemeData(
        brightness: Brightness.dark,
        appBarTheme: AppBarTheme(
          centerTitle: true,
          toolbarHeight: 150, // Change to 150.
          shadowColor: Colors.yellow,
          elevation: 5, // Add an elevation to make shadow visible.
          shape: RoundedRectangleBorder(
            borderRadius: BorderRadius.circular(50),
          ),
        ),
        floatingActionButtonTheme: const FloatingActionButtonThemeData(
          backgroundColor: Colors.yellow,
          /*
           * The "+" in the action button defaults to a light color,
           * which makes it hard to see against the yellow background color.
           * So, I changed the [foregroundColor].
           */
          foregroundColor: Colors.black,
        ),
        fontFamily: 'Georgia',
        textTheme: TextTheme(
          // There's no [headline4].
          headlineMedium: TextStyle(fontSize: 36, fontStyle: FontStyle.italic),
        ),
        /*
         * The default template adds a purple [colorScheme]. This conflicts with
         * [brightness], so I removed it.
         */
      ),
      home: const MyHomePage(title: 'Flutter Demo Home Page'),
    );
  }
}

class MyHomePage extends StatefulWidget {
  const MyHomePage({super.key, required this.title});

  final String title;

  @override
  State<MyHomePage> createState() => _MyHomePageState();
}

class _MyHomePageState extends State<MyHomePage> {
  int _counter = 0;

  void _incrementCounter() {
    setState(() {
      _counter++;
    });
  }

  @override
  Widget build(BuildContext context) {
    return Scaffold(
      appBar: AppBar(
        backgroundColor: Theme.of(context).colorScheme.inversePrimary,
        title: Text(widget.title),
      ),
      body: Center(
        child: Column(
          mainAxisAlignment: MainAxisAlignment.center,
          children: <Widget>[
            const Text('You have pushed the button this many times:'),
            Text(
              '$_counter',
              style: Theme.of(context).textTheme.headlineMedium,
            ),
          ],
        ),
      ),
      floatingActionButton: FloatingActionButton(
        onPressed: _incrementCounter,
        tooltip: 'Increment',
        child: const Icon(Icons.add),
      ), // This trailing comma makes auto-formatting nicer for build methods.
    );
  }
}