Widget build or Function return?

Question

I'm relatively new to coding with flutter and using widgets and I have a simple query. Here are 2 code snippets that have my themeData that I want to return to the theme property in my MaterialApp. The snippet is in a different file to the main.dart file and is imported in. Here's how it'll fit in.

void main() {
  runApp(MaterialApp(
    title: 'App Name',
    theme: AppTheme(),
    home: MyApp(),
  ));
}

Now my query is... Which is a better practice; Using a function to return a widget or creating another widget to parent that widget and return that?

ThemeData AppTheme(){
  return ThemeData(
    ...
  );
}

OR

class AppTheme extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    return ThemeData(
      ...
    );
  }
}