add a class in css
.dropdown
{
border: 1px solid #ccc;
-webkit-border-radius: 5px;
-moz-border-radius: 5px;
border-radius: 10px;
padding: 2px 3px;
cursor: text;
height: 30px;
width: 270px;
cursor: pointer !important;
}
<asp:dropdownlist id="ddlSomething" cssclass="dropdown" runat="server" xmlns:asp="#unknown">
</asp:dropdownlist>