In order to add the table of contents a <div> with an id of table-of-contents was added to the article-layout.html file. It displays the PDF template variable {{t_of_c}}.

The following CSS is then added to the Custom CSS tab. The classes referenced in the CSS are output by the {{t_of_c}} template variable.

#table-of-contents {
  /* Name the page with this div so we can target header/footer and other props with @page */
  page: table-of-contents;
  
  /* So we don't get too close to the header/footer */
  padding-bottom: 8pt;
}

#table-of-contents .table-of-contents-title {
  /* The text for the <h1> tag displaying the t of c title */
  content: 'My Table of Contents';
}

#table-of-contents a {
  text-decoration: none;
  color: rgb(0,0,0);
}

#table-of-contents .step-tofc {
}

#table-of-contents .sub-step-tofc {
  margin-left: 50px;
}

#table-of-contents a::after {
  content: leader(".") target-counter(attr(href), page);
}