The Styles.Links.Link:visited section controls the styles applied to a link when the link has already been visited by the current visitor.
I do not recommend using this style to alter the appearance of visited links. The effect applies at a page level, but some content on the page may not have been visible when the user last visited it. For example, if the user clicked on a link to person 10, a link to person 11 (on the same page) would appear in the visited state, but person 11 was not visible when the visitor saw that page.
See the Styles Section help page for more details.