Código atualizado 2022

Fiz o código baseado em um outro post, porém troquei o background color do app bar pelo primarySwatch: primaryGreen(),do themer

theme: ThemeData(
        primaryColor: Colors.green[800],
        colorScheme: ColorScheme.fromSwatch(
          primarySwatch: primaryGreen(),
        ).copyWith(
          secondary: Colors.blueAccent[700],
        ),
        buttonTheme: ButtonThemeData(
          buttonColor: Colors.blueAccent[700],
          textTheme: ButtonTextTheme.primary,
        ),
      ),

MaterialColor primaryGreen() {
  return MaterialColor(
    0xFF2E7D32,
    {
      50: Colors.green[50]!,
      100: Colors.green[100]!,
      200: Colors.green[200]!,
      300: Colors.green[300]!,
      500: Colors.green[500]!,
      600: Colors.green[600]!,
      700: Colors.green[700]!,
      800: Colors.green[800]!,
      900: Colors.green[900]!,
    },
  );
}