Need help setting up tab selector

I have noticed that WP inserts a “current-page-item” on list menus when you are on a selected page. I am trying to use this element in a theme. I want a menu tab to change color when the user is on the selected page. I have tried a bunch of code combinations, but I cannot get the tab color to change.

I have pasted the CSS code below and the site is located at http://www.wasatchdeli.us

I am sure there is a way to do this, I just haven’t found it yet.

.topnav ul {
     margin: 8px 0 5px 0;
     font-family: ariel, helvetica, sans-serif;
     font-size: 16px;
     font-weight: bold;
}

.topnav ul li {
     display: inline;
     padding-left: 15px;
}

.topnav ul li a {
     padding: 5px 10px 5px 10px;
     text-decoration: none;
     background: #42420B;
     color: #fff;
     border-top-left-radius: 5px;
     border-top-right-radius: 5px;
}

.topnav ul li a:hover {
     background: #fff;
     padding: 5px 10px 5px 10px;
     text-decoration: none;
     color: #000;
     border-top-left-radius: 5px;
     border-top-right-radius: 5px;
}

.topnav li.current-page-item a {
     background: #fff;
     padding: 5px 10px 5px 10px;
     text-decoration: none;
     color: #000;
     border-top-left-radius: 5px;
     border-top-right-radius: 5px;
}