Cannot set background color of ComboBox in Eto Forms

Scripting
15

I see. Many thanks for the answer @TomTom

I have another post here asking for help but didn’t get any reply

   Styles.Add<Eto.Wpf.Forms.Controls.GroupBoxHandler>("secondaryBox", h => { 
                h.Control.Background = System.Windows.Media.Brushes.Black;
                h.Control.BorderThickness = new System.Windows.Thickness(5);
                h.Control.BorderBrush = System.Windows.Media.Brushes.Red;
            });

I am trying to change the border’s color with Wpf.
It seems to me that I can’t change these unexposed style if I stick to Eto?