.example/private/layout.html

<html xmlns="http://www.w3.org/1999/xhtml">
  <head>
    <meta http-equiv="Content-Type" content="text/html; charset=UTF-8" />
    <meta name="viewport" content="width=device-width, initial-scale=1.0" />

    <title>{{preview}}</title>

    <style type="text/css">
      {{{css}}}
    </style>
  </head>
  <body leftmargin="0" marginwidth="0" topmargin="0" marginheight="0" offset="0">

    <!-- start body table -->
    <table cellspacing="0" cellpadding="0" border="0" height="100%" width="100%" class="body-table">
      <tr>
        <td class="body" align="center" valign="top">

          <p class="hide preview-text">{{#if preview}}{{preview}}{{else}}Default preview.{{/if}}</p>

          <!-- begin all -->
          <table cellspacing="0" cellpadding="0" border="0" width="100%">

            <!-- start header -->
            <tr class="header"><td align="center">

              <table cellspacing="0" cellpadding="0" border="0" class="container">
                <tr>
                  <td class="header__content" valign="top" align="center">

                    <h1>Header</h1>

                  </td>
                </tr>
              </table>
            </td></tr><!-- end header -->

            <!-- MAIN TEMPLATE CONTENT -->

            <tr class="template-content">
              <td align="center">
                {{{ body }}}
              </td>
            </tr>

            <!-- end MAIN TEMPLATE CONTENT -->


            <!-- begin footer -->
            <tr class="footer">
              <td>
                <p><small>Some footer content</small></p>
              </td>
            </tr>
            <!-- end footer -->

          </table><!-- end -->
        </td>
      </tr>
    </table><!-- end body table -->

  </body>
</html>