combobox background programmatically

Question

hello please i'm trying to change color of combobox itry with backgroun property

combo.Background= (Brush)new BrushConverter().ConvertFrom("#FEFCD7"); 

but just border changed 

i even try to reach all component inside the combo via VisualTreeHelper and i find that it's composed of Grid then togglebutton and contentPresenter then under the contentpresenter there is textblock i try to change the textblock background but it doesn't work also the grid background but no result 

DependencyObject child = VisualTreeHelper.GetChild(combo, 0);

(child as Grid).Background = (Brush)new BrushConverter().ConvertFrom("#FEFCD7");

int x = VisualTreeHelper.GetChildrenCount(child);

List<DependencyObject> childs = new List<DependencyObject>();

for (int i = 0; i < x; i++)

{

DependencyObject child2=VisualTreeHelper.GetChild(child, i);

childs.Add(child2);

if (child2.GetType() == typeof(ContentPresenter))

{

DependencyObject child3 = VisualTreeHelper.GetChild(child2, 0 );

int c = VisualTreeHelper.GetChildrenCount(child2);

(child3 as TextBlock).Background = Brushes.Transparent;

(child3 as TextBlock).Background = (Brush)new BrushConverter().ConvertFrom("#FEFCD7");

}

}

please i don't want to change highlightind just the background ,  i'm using the win8 theme for all my windows