lib/rdoc/generator/template/darkfish/_sidebar_classes.rhtml

<div id="classindex-section" class="nav-section">
  <h3>Class and Module Index</h3>

  <ul class="link-list">
  <%- @modsort.each do |index_klass| -%>
    <li><a href="<%= rel_prefix %>/<%= index_klass.path %>"><%= index_klass.full_name %></a>
  <%- end -%>
  </ul>
</div>