add an id
attribute to the body
tag of each of your pages, like this:
<body id="home"> <!-- this would be for your home page for example -->
...
<body id="about"> <!-- this would be for your about page... -->
add the same to the li
tags of your nav, like this:
<li id="home">Home</li>
<li id="about">About</li>
then in the CSS file just do this:
body#home li#home, body#about li#about { // style of the active menu item }