MetroDialog with custom background color

I want to display a MetroDialog with a custom background color, for error messages.

I tried using the property CustomResourceDictionary without luck:

Function to display message:

public static async Task<MessageDialogResult> DialogOK(string title, string message)
{
    MetroDialogSettings settings = new MetroDialogSettings();
    settings.CustomResourceDictionary = new ResourceDictionary { Source = new Uri("pack://application:,,,/MyApp;component/Resources/ErrorDialogRD.xaml") };
    var result = await mainWindow.ShowMessageAsync(title, message, MessageDialogStyle.Affirmative, settings );
    return result;
}

Resource dictionary:

<ResourceDictionary xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
                    xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
                    xmlns:Dialogs="clr-namespace:MahApps.Metro.Controls.Dialogs;assembly=MahApps.Metro">
    <Style TargetType="{x:Type Dialogs:BaseMetroDialog}">
        <Setter Property="Background" Value="Red" />
    </Style>
</ResourceDictionary>

What am I missing? :)