var appTheme = ThemeData.from(colorScheme: ColorScheme.light()).copyWith(
  appBarTheme: AppBarTheme(
    titleTextStyle: TextStyle(
      fontSize: 18,
      fontWeight: FontWeight.w600,
      color: AppColors.primaryText,
    ),
  ),
// ...
);

class MyApp extends StatelessWidget {
  // This widget is the root of your application.
  @override
  Widget build(BuildContext context) {
    return ScreenUtilInit(
        designSize: Size(375, 667),
        builder: () => GetMaterialApp(
              theme: appTheme,
              initialRoute: AppRoutes.index,
              getPages: AppPages.pages,
            ));
  }
}

return Scaffold(
      appBar: AppBar(
       // but the style set by titleTextStyle is now work!!!
        title: Text(
          'test',
        ),
      ),
      body: _buildBody(),
    );