static MainWindow()
{
	EventManager.RegisterClassHandler(typeof(RadComboBox), RadComboBox.LoadedEvent, new RoutedEventHandler(OnComboBoxLoaded));
}

private static void OnComboBoxLoaded(object sender, RoutedEventArgs e)
{
	var comboBox = (RadComboBox)sender;
	if (!comboBox.IsEditable)
	{
		if (comboBox.IsEnabled)
		{
			var buttonChrome = comboBox.FindChildByType<ButtonChrome>();
			buttonChrome.Background = Brushes.Purple;
		}
		else
		{
			comboBox.Background = Brushes.Purple;
		}                
	}	
}