From their examples:

    <article class="mw7 center ph3 ph5-ns tc br2 pv5 bg-washed-green dark-green mb5">
        <a class="f6 br-pill bg-dark-green no-underline washed-green ba b--dark-green grow pv2 ph3 dib mr3"href="#">Sign Up</a>
    </article>

...I sure wouldn't want to maintain that site.