title: Google Apps Script - template statements to include external files not working
body: <p>I'm trying to use external CSS stylesheet.  However what I get is the '<strong>* include stylesheet *</strong>'  rendering in my html sidebar.  I created the include function in my code.gs file.  Why is this not working? </p>
  
  <p><strong>Index.html</strong></p>
  
  <pre><code>&lt;!DOCTYPE html&gt;
  &lt;html&gt;
   &lt;head&gt;
   &lt;base target="_top"&gt;
   &lt;?!= include('stylesheet'); ?&gt;
   &lt;/head&gt;
   &lt;body&gt;
    &lt;script
     src="//ajax.googleapis.com/ajax/libs/jquery/1.9.1/jquery.min.js"&gt;
    &lt;/script&gt;
   &lt;/body&gt;
  &lt;/html&gt;
  </code></pre>
  
  <p><strong>Code.gs</strong></p>
  
  <pre><code>function onOpen() {
   SpreadsheetApp.getUi()
   .createMenu('Form')
   .addItem('Open', 'openSidebar')
   .addToUi();
  }
  function include(filename) {
   return HtmlService.createHtmlOutputFromFile(filename)
    .getContent();
  }
  function openSidebar(){
   var htmlOutput = HtmlService.createHtmlOutputFromFile('Index');
   htmlOutput.setTitle('Customer Inquiry Form');
   SpreadsheetApp.getUi().showSidebar(htmlOutput);
  }
  </code></pre>
  
  <p><strong>stylesheet.html</strong></p>
  
  <pre><code>&lt;style&gt;
   body{
    background-color:'gray';
   }
  &lt;/style&gt;
  </code></pre>