First, it'll be a lot easier (and easier for your customers to read) if you increase the width of the left column so your category names don't wrap. Then you can find this in the stylesheet and add the red parts:

A.category-top, A.category-top:visited {
color: #008000;
text-decoration: none;
border-bottom:1px solid;
line-height:1.5em;
}


If you want the category name text to change color on hover, add this to the stylesheet:

A.category-top:hover {
color: #ff0000;
}