Css Code
.autocomplete_list
{
visibility: hidden;
margin: 0px !important;
padding: 0px;
background-color: inherit;
color: windowtext;
border: buttonshadow;
border-width: 1px;
border-style: solid;
cursor: default;
text-align: left;
list-style-type: none;
overflow: auto;
font-weight: normal;
font-size: 11px;
z-index: 100;
font-family: Arial;
}
.autocomplete_highlighted_listitem
{
background-color: Highlight;
color:Red;
padding: 3px;
font-family: Arial;
}
.autocomplete_listItem
{
background-color: window;
color:blue;
padding: 50px;
font-family: Arial;
z-index: 1000;
position:static;
}
<html>
<head></head>
<body>
<table>
<tr>
<td><pre><asp:TextBox CssClass="textbox_SixColumns" BackColor="White" runat="server" ID="TX_USR_UName_A"
AutoPostBack="true" OnTextChanged="TX_USR_UName_A_TextChanged"></asp:TextBox>
<br />
<asp:RequiredFieldValidator ID="RF_TX_USR_UName_A" runat="server" ControlToValidate="TX_USR_UName_A"
CssClass="RedMessage" Display="Dynamic" ErrorMessage="Please enter username."
ValidationGroup="VG_Add_U"></asp:RequiredFieldValidator>
<ajaxToolkit:TextBoxWatermarkExtender ID="AXW_Search" runat="server" TargetControlID="TX_USR_UName_A"
WatermarkCssClass="watermarkPopup" WatermarkText="Search...">
</ajaxToolkit:TextBoxWatermarkExtender>
<ajaxToolkit:AutoCompleteExtender ID="AXAC_AutoSearch" runat="server" MinimumPrefixLength="1"
ServiceMethod="GetBCSearchAutoCompleteUsers" ServicePath="~/SYS/Sunray/AutoComplete.asmx"
BehaviorID="bhText" UseContextKey="true" ContextKey="a" CompletionInterval="50"
TargetControlID="TX_USR_UName_A" CompletionSetCount="10" CompletionListCssClass="autocomplete_list"
CompletionListItemCssClass="autocomplete_listItem" CompletionListHighlightedItemCssClass="autocomplete_highlighted_listitem">
</ajaxToolkit:AutoCompleteExtender></td>
</tr>
</table>
</body>
</html>