BuddyPress.org

What CSS did you write?

You’ll need something like:

ul#topic-post-list li:first-child {
…
}

If that doesn’t work, which browser are you using? If you are on an old browser, the first-child selector might not work.

Check here for more info on first-child: http://reference.sitepoint.com/css/pseudoclass-firstchild

Note for development: Firefox (with firebug extension) or Safari with the developer menu is a good choice… you can view (and experiment) with css direct in the browser. Once you’re happy, you can then test in IE.