如何给DropDownList控件添加边框

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<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>

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