>>>
class UnitConverterApp extends StatelessWidget {
  Widget build(BuildContext context) {
    return MaterialApp(
      theme: ThemeData(
        textTheme: Theme.of(context).textTheme.apply(
          bodyColor: Colors.black,
          displayColor: Colors.grey[600],
        ),
      ),
    );
  }
}
<<<
class UnitConverterApp extends StatelessWidget {
  Widget build(BuildContext context) {
    return MaterialApp(
      theme: ThemeData(
        textTheme: Theme.of(context).textTheme.apply(
          bodyColor: Colors.black,
          displayColor: Colors.grey[600],
        ),
      ),
    );
  }
}