If you're editing a skin someone else made, unfortunately you have to use a css editing add-on like stylish for firefox, etc, or get all the colors and assets for the skin from the creator so you can edit them yourself. If it's in a skin you've made, the colors you edit are these here. The link color is a regular unclicked link (the target url isn't in your history), the visited link is a clicked like (target url is in your history), and active link is the color it turns right as you click the link.