How to change the Background of tooltip in dropdownlist in asp.net c#

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C#

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SQL-Server

How to change the Background of tooltip in dropdownlist in asp.net c# ??
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I want to change the Background of item, which is mouseover in this dropdownlist items...


This is my code..

private void Fill_Colleges()
    {

        SqlConnection con = new SqlConnection(_connString);
        con.Open();
       SqlCommand cmd = new SqlCommand("select * from Colleges ORDER BY College_Code", con);
      
       
        SqlDataAdapter da = new SqlDataAdapter(cmd);
        DataSet ds = new DataSet();
        da.Fill(ds);
        DDLColleges.DataSource = ds;
        DDLColleges.DataTextField = "College_Code";
        DDLColleges.DataValueField = "CollegeName";
        DDLColleges.DataBind();

       DDLColleges.Items.Insert(0, new System.Web.UI.WebControls.ListItem("--------- Select ---------", "0"));

        con.Close();


    }

protected void DDLColleges_DataBound(object sender, EventArgs e)
    {

        DropDownList ddl = sender as DropDownList;
        if (ddl != null)
        {
            foreach (ListItem li in ddl.Items)
            {
                li.Attributes["Title"] = li.Value.ToUpper();

                li.Attributes.Add("style", "color:#1B3E74");

            }
        }
    }

Please help me, thanks.