#ある日、titleがdeprecatedになっていた。。
Flutterのstyleで使うtitleが~~title~~になっていました。。

class MyApp extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    initializeDateFormatting('ja');
    return MaterialApp(
      title: 'Persona Expenses',
      theme: ThemeData(
          primarySwatch: Colors.red,
          accentColor: Colors.amber,
          fontFamily: 'Quicksand',
          textTheme: ThemeData.light().textTheme.copyWith(
              title: TextStyle(
                  fontFamily: 'OpenSans',
                  fontSize: 18,
                  fontWeight: FontWeight.bold)),
      home: MyHomePage(),
    );
  }
}

#headline6を使いましょう

class MyApp extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    initializeDateFormatting('ja');
    return MaterialApp(
      title: 'Persona Expenses',
      theme: ThemeData(
          primarySwatch: Colors.red,
          accentColor: Colors.amber,
          fontFamily: 'Quicksand',
          textTheme: ThemeData.light().textTheme.copyWith(
             // 変更！
              headline6: TextStyle(
                  fontFamily: 'OpenSans',
                  fontSize: 18,
                  fontWeight: FontWeight.bold)),
      home: MyHomePage(),
    );
  }
}

#参考