How do I make one tab in primary nav (Zen-based theme) look different?

Hi,

I'm trying to change the background color of one of the Primary nav bar tabs within the Zen-based theme.

#block-menu-primary-links ul li.special {
background: none repeat scroll 0 0 transparent;
float: left;
margin: 0;
padding: 0;
}

I've identified a class for this one tab in layout.css (above)- but how do I let Drupal know that this should be class "special?" Is there anything I should put in the CSS code to let Drupal know I'm referring to the 5th li, for example, or the li with the particular, content, text or href?

I appreciate your help very much!

-gg

ps I'm able to make the changes, playing around in Firebug, but again, I need to change the system code on the files in the server.