[Ted S]

I can't tell you where to place it as I haven't poked around enough but I already told you the template (postbit or postbig_legacy depending on your settings).

As far as how to use it,

<td class="alt1" <if condition="$threadinfo[toppost] == $post[postid]">style="background-color:red;"</if> >

Would make a table cell (<td>) colored red if it's the top post.

I'd suggest opening the right template and just playing. You can create a new style as a child and edit that so your members don't see what you're doing until it's complete.