<style type="text/css">
        .CustomerComboBox .ajax__combobox_inputcontainer
        {
        }
        .CustomerComboBox .ajax__combobox_textboxcontainer
        {
        }
        .CustomerComboBox .ajax__combobox_textboxcontainer input
        {
            margin: 0;
            border: solid 1px #7F9DB9;
            border-right: 0px none;
            padding: 1px 0px 0px 5px;
            font-size: 13px;
            height: 18px;
            position: relative;
        }
        .CustomerComboBox .ajax__combobox_buttoncontainer
        {
        }
        .CustomerComboBox .ajax__combobox_buttoncontainer button
        {
        }
        .CustomerComboBox .ajax__combobox_itemlist
        {
            /*  position: absolute; */
            top: auto;
            width: 500px;
            height: 100px;
            position: relative;
            border: 1px yellow solid;
        }
        .CustomerComboBox .ajax__combobox_itemlist li
        {
            /*width: 500px; */
            height: inherit;
            overflow: inherit;
            border: 1px yellow solid;
           
        }
    </style>

<cc1:ComboBox ID="ComboBox2" runat="server" CssClass="CustomerComboBox" DropDownStyle="DropDownList"
                    AutoCompleteMode="Append" CaseSensitive="False">
                    <asp:ListItem>1    john</asp:ListItem>
                    <asp:ListItem>2    john</asp:ListItem>
                    <asp:ListItem>3    john</asp:ListItem>
                    <asp:ListItem>4    john</asp:ListItem>
                    <asp:ListItem>5    john</asp:ListItem>
                    <asp:ListItem>6    john</asp:ListItem>
                    <asp:ListItem>7    john</asp:ListItem>
                    <asp:ListItem>8    john</asp:ListItem>
                    <asp:ListItem>9    john</asp:ListItem>
                    <asp:ListItem>10john</asp:ListItem>
                    <asp:ListItem>11john</asp:ListItem>
                    <asp:ListItem>12john</asp:ListItem>
                    <asp:ListItem>13john</asp:ListItem>
                    <asp:ListItem>14john</asp:ListItem>
                    <asp:ListItem>15john</asp:ListItem>
                    <asp:ListItem>16john</asp:ListItem>
                    <asp:ListItem>17john</asp:ListItem>
                    <asp:ListItem>18john</asp:ListItem>
                    <asp:ListItem>19john</asp:ListItem>
                    <asp:ListItem>20john</asp:ListItem>
                    <asp:ListItem>21john</asp:ListItem>
                    <asp:ListItem>22john</asp:ListItem>
                    <asp:ListItem>23john</asp:ListItem>
                    <asp:ListItem>24john</asp:ListItem>
                    <asp:ListItem>25john</asp:ListItem>
                    <asp:ListItem>26john</asp:ListItem>
                    <asp:ListItem>27john</asp:ListItem>
                    <asp:ListItem>28john</asp:ListItem>
                    <asp:ListItem>29john</asp:ListItem>
                    <asp:ListItem>30john</asp:ListItem>
                    <asp:ListItem>31john</asp:ListItem>
                    <asp:ListItem>32john</asp:ListItem>
                    <asp:ListItem>33john</asp:ListItem>
                    <asp:ListItem>34john</asp:ListItem>
                    <asp:ListItem>35john</asp:ListItem>
                </cc1:ComboBox>

Item is not highted when i pressed down arrow.

pls help