data8/_includes/sidebar-toggle.html

<div class="c-sidebar-toggle">
  <!-- We show the sidebar by default so we use .is-active -->
  <button
    id="js-sidebar-toggle"
    class="hamburger hamburger--arrowalt is-active"
  >
    <span class="hamburger-box">
      <span class="hamburger-inner"></span>
    </span>
    <span class="c-sidebar-toggle__label">Toggle Sidebar</span>
  </button>
</div>