WpRecipes.com - Daily recipes to cook with WordPress

When you're browsing a category, WordPress automatically add a current-cat css class to the <li> element.

So the only thing you have to do is to edit your style.css file and add a style for the current-cat class:

#nav .current-cat{
    background:#999;
    color:#222;
    text-decoration:underline;
}

That's all! Now your readers will know exactly what they're browsing right now.