AjaxControlToolkit, ComboBox style

I am with a little problem with the arrow image of a ComboBox control (AjaxControlToolkit).

I define this style:

.WindowsStyle .ajax__combobox_inputcontainer .ajax__combobox_buttoncontainer button
{
    margin: 0;
    padding: 0;
    background-image: url(../icons/windows-arrow.gif);
    background-position: top left;
    border: 0px none;
    height: 21px;
    width: 21px;
}

I set this style on combobox, but the control are showing the border of the textbox before the arrow:

Look here, the border aren't showing!

How can I hide this border?

This is working fine for me, I have the following css in my page head (though it can go anywhere else like in a stylesheet of course)

<style type="text/css">
    .WindowsStyle .ajax__combobox_inputcontainer .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;       
    }
    .WindowsStyle .ajax__combobox_inputcontainer .ajax__combobox_buttoncontainer button
    {
        margin: 0;
        padding: 0;
        background-image: url(windows-arrow.gif);
        background-position: top left;
        border: 0px none;
        height: 21px;
        width: 21px;
    }
    .WindowsStyle .ajax__combobox_itemlist
    {
        border-color: #7F9DB9;
    }
</style>

And then I have this control markup in the body of my page:

<ajaxToolkit:ComboBox ID="ComboBox1" runat="server" CssClass="WindowsStyle">
<asp:ListItem Text="[Select an item]" Value="" />
<asp:ListItem Text="Actual Item #1" Value="SomeValue" />
<asp:ListItem Text="Actual Item #2" Value="3" />
<asp:ListItem Text="Actual Item #3" Value="xxx" />
</ajaxToolkit:ComboBox>

Perhaps you have some conflicting styling? Have you tried a simple page with nothing but what you require to generate the combobox?