如何给DropDownList控件添加边框

<span style="border-right: gray 1px solid; border-top: gray 1px solid;
                border-left: gray 1px solid; border-bottom: gray 1px solid;">

<asp:DropDownList ID="ddlSearch" runat="server">
                    <asp:ListItem Value="title">标题</asp:ListItem>
                    <asp:ListItem Value="content">内容</asp:ListItem>
                    <asp:ListItem Value="author">作者</asp:ListItem>
                </asp:DropDownList>

</span>