public async void Button_Click(object sender, RoutedEventArgs e)
{
   var metroWindow = (Application.Current.MainWindow as MetroWindow);
   await metroWindow.ShowMessageAsync("Title", "Body");
}

<ResourceDictionary Source="pack://application:,,,/MahApps.Metro;component/Styles/Accents/Cobalt.xaml" />

metroWindow.MetroDialogOptions.ColorScheme = MetroDialogOptions.ColorScheme;