Add style for first item in RadComboBox

[Grzegorz]

Hello,

How add style for first item in RadComboBox on the side server?How get first element RadComboBox, element type input ?

protected void ValidationSections()
{

     if (RadComboBox.Text == "")
     {
         RadComboBox.Style.Add("background", "#F7FAA5 !important;"); //not working
         RadComboBoxItem item = RadComboBox.DataValueField(""); // error Non-invocable member '.../DataValueField' cannot be used like a method
         MainContent_RadComboBox_Input.Style.Add("background", "#F7FAA5 !important;"); //  MainContent_RadComboBox_Input.Style not exist in the current context
     }
}

Please help me.