The following worked for me.
HTML:
<asp:DropDownList ID="DropDownList1" CssClass="DropDownList" runat="server">
<asp:ListItem>first</asp:ListItem>
<asp:ListItem>second</asp:ListItem>
<asp:ListItem>third</asp:ListItem>
<asp:ListItem>fourth</asp:ListItem>
</asp:DropDownList>
CSS:
.DropDownList
{
width:61%;
border:1px solid #7f9db9;
vertical-align:middle;
height:32px;
padding:5px;
}
.DropDownList option
{
border-bottom:1px solid #7f9db9;
padding:5px;
vertical-align:middle;
}
.DropDownList option:last-child
{
border-bottom:none;
}