In your template directory, create a file names style.css, if it doesn't already exist. Then look at design.css in the links section, where all of the css for links is set up. What you want to change is the style for a:wikilink1:visited. You define this style ins style.css. For instance
a:wikilink1:visited {
color: blue;
}
You might mark it !important:
color: blue !important;