popup_menu
popup_menu_content
popup_menu_header
popup_menu_highlight
Those style all your popups. sooo, for example giz wanted to make the catagories stand out from the forums/sub-forums.
he could style the categories with class="popup_menu_header" and the others with "popup_menu_content"
they are all in "Popup menu properties" section of the styles editor in the cpanel.
Also, i'll bet you are using IE ?