Question: How to change Combobox.Background?
Since Windows 8 
`ComboBox.Background=Brushes.Red` 
has no effect on ComboBox

Answer: 
I stumbled into the same problem and it brought me here.

Alex's answer gave me an idea, and after looking into the Windows 8 ComboBox's control template, I came to this solution:

```
private void ComboBox_Loaded(Object sender, RoutedEventArgs e)
{
    var comboBox = sender as ComboBox;
    var comboBoxTemplate = comboBox.Template;
    var toggleButton = comboBoxTemplate.FindName("toggleButton", comboBox) as ToggleButton;
    var toggleButtonTemplate = toggleButton.Template;
    var border = toggleButtonTemplate.FindName("templateRoot", toggleButton) as Border;

    border.Background = new SolidColorBrush(Colors.Red);
}

```

The plus side of this solution is that it's pretty straightforward, but it comes with a drawback: it seems to override all states, so there's little visual feedback on mouse over and such. I will edit my answer if I come up with a better idea.

Just make sure to add some error checking, as this can, and probably will, fail as soon as the code is executed on a system other than Windows 8.x, or if Microsoft ever attempts to fix the issue in question and modifies the template.
I know it's late, but I have found some way to fix it. Just use corrected ComboBox.

```
using System.Windows.Controls;
using System.Windows;
using System.Windows.Data;
namespace Utils {
    class ComboBoxWin8 : ComboBox {
        public ComboBoxWin8() {
            Loaded += ComboBoxWin8_Loaded;
        }
        void ComboBoxWin8_Loaded(object sender, RoutedEventArgs e) {
            ControlTemplate ct = Template;
            Border border = ct.FindName("Border", this) as Border;

            // if Windows8
            if (border != null) {
                border.Background = Background;

                // In the case of bound property
                BindingExpression be = GetBindingExpression(ComboBoxWin8.BackgroundProperty);
                if (be != null) {
                    border.SetBinding(Border.BackgroundProperty, be.ParentBindingBase);
                }
            }
        }
    }
}

```