It's the same button that styles the buttons within the search box and a few other areas (not Post Reply, New Thread, though)
.button and .button:hover and .button:active
Or you can do it via SP, but it's not as much fun to style
Doesn't work (and the SP configuration doesn't either).... but I found that it has to do with the Ajax Conversation in Postbit add-on. I styled it in CSS but apparently it stomps on these also. When I get a chance I've got to post the info in the add-on's support topic.