Workaround:
void radDropDownList1_VisualListItemFormatting(object sender, VisualItemFormattingEventArgs args)
        {
            if (args.VisualItem.Selected && radDropDownList1.ThemeName == "VisualStudio2012Dark")
            {
                args.VisualItem.BackColor = Color.DarkBlue;
            }
            else
            {
                args.VisualItem.ResetValue(LightVisualElement.BackColorProperty, ValueResetFlags.Local);
            }
        }