You either have to use ThemeManager.Current.ThemeSyncMode = ThemeSyncMode.SyncWithAppMode; (you can choose whichever ThemeSyncMode you like of course) before calling SyncTheme or you have to pass a sync mode to SyncTheme.

Thank you. But It's not working as expected yet. I have the following Code:

`public partial class MainWindow : MetroWindow
{
public MainWindow()
{
InitializeComponent();
ThemeManager.Current.ThemeSyncMode = ThemeSyncMode.SyncWithAppMode;
ThemeManager.Current.ChangeTheme(this, "Light.Blue");
ThemeManager.Current.SyncTheme();
}

private void Button_Click(object sender, RoutedEventArgs e)
{
    new SecondWindow().Show();
}

private void ChangeTheme_Click(object sender, RoutedEventArgs e)
{
    ThemeManager.Current.ChangeTheme(this, "Dark.Blue");
    ThemeManager.Current.SyncTheme();
}

}`
and

` public partial class SecondWindow : MetroWindow
{
public SecondWindow()
{
ThemeManager.Current.SyncTheme();
InitializeComponent();

 }

 private void Button_Click(object sender, RoutedEventArgs e)
 {
     ThemeManager.Current.ChangeTheme(this, "Dark.Blue");
     ThemeManager.Current.SyncTheme();
 }

}`

When I run the application, the MainWindow appears in Light.Blue, as expected. When I click on the Button to show the second window, it appears totaly black. I can not see the buttons on this window. I don't understand why?

I expect that the SecondWindow would also appear in Light.Blue. The ChangeTheme-Buttons on the windows work as expected. The Theme is changing.

Thank you for the quick answer. This did the trick. I also removed all calls of SyncTheme(). Apparently it works without it. Why is this method needed?

Sorry, I see the answer is in your first sentence.