Since all Aspx controls rendered as html controls you can write css as below..
css below..
<style type="text/css">
select
{
color:#FFFFFF;
margin:0px 0px 0px 0px !important;
padding:0px 0px 0px 0px !important;
width:200px;
}
select option
{
background:#000;
}
</style>
Html below..
<asp:dropdownlist id="ddlTest" runat="server" xmlns:asp="#unknown">
<asp:listitem value="Item 1">Item 1</asp:listitem>
<asp:listitem value="Item 2">Item 2</asp:listitem>
</asp:dropdownlist>