Do you mean that you want to display the selected HyperLink with a Green background? If so, you need to modify the attributes of the Link. However, once a HyperLink has been selected or visited, the background color would remain as Green no matter what.
<asp:bulletedlist id="BulletedList1" runat="server" cssclass="BulletedList" displaymode="HyperLink"
target="_blank">
<asp:listitem text="Microsoft" value="http:\\www.microsoft.com" />
<asp:listitem text="ESPN" value="http:\\espn.com" />
</asp:bulletedlist>
.BulletedList A:link { background-color: Red; color: White; }
.BulletedList A:visited { background-color: Green; color: White; }
.BulletedList A:hover { background-color: Blue; color: White; }