[Rowsdower!]

You can assign a class to the <a> tag when it is the "current" page. You can even assign multiple classes to a single element in the HTML if you need to. Then you just make a CSS rule to style up the "current" link. It can be done any number of ways, but in its most basic form it might look something like this:

HTML:

<div class="sf_navigation_top">
  <ul>
    <li><a href="">Link Text Here</a></li>
    <li><a href="" class="current">Link Text Here</a></li>
    <li><a href="">Link Text Here</a></li>
    <li><a href="">Link Text Here</a></li>
    <li><a href="">Link Text Here</a></li>
  </ul>
</div>

CSS:

.sf_navigation_top ul li a {
text-decoration: none;
display: block;
font-family: Verdana, Arial, Helvetica, sans-serif;
font-size: 14px;
font-weight: bold;
font-style: normal;
font-variant: normal;
text-transform: none;
padding: 10px 19px 10px 19px;
}

.sf_navigation_top ul li a.current {
background-color:#f00;
}

If you can provide a link to your test page I can provide a more specific answer to the question.