Most readers like to know where they are on a website. To help them knowing what they’re exactly browsing right now, you should definitely highlight the current category. It is very easy to do, just read on.
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.