<!DOCTYPE html>
<html>
  <head>
    <style>
        .link {
          color: var(--link-color);
        }
        .link:visited {
          color: var(--link-color-visited);
        }
        .theme {
          --link-color: green;
          --link-color-visited: red;
        }
    </style>
  </head>
  <body>
    <a href="https://not-a-site.com" class="link theme">Link</a>
  </body>
</html>