jQuery UI dialog in a userscript for Greasemonkey - missing close button

Question

I have a userscript (for Greasemonkey) where I use a jQuery UI dialog. The problem is that the close button appears empty.

My code was like this:

// ==UserScript==
// @name        test
// @grant       GM_addStyle
// @grant       GM_getResourceText
// @require     http://code.jquery.com/jquery-2.1.4.min.js
// @require     http://code.jquery.com/ui/1.11.4/jquery-ui.min.js
// @resource    source_2 http://code.jquery.com/ui/1.11.4/themes/smoothness/jquery-ui.css  
// ==/UserScript==

var src2 = GM_getResourceText ("source_2");
GM_addStyle (src2);

...


$(function() {
  $( "#dialog" ).dialog({
    title: 'My dialog'
  });
});

In order to get the (full) jQueryUI theme's URL so that I may include it as a @resource in my script,
(i.e. visit this page and select Smoothness theme on bottom of the page)
it just prompts to download a whole .zip file - which I can't use in my userscript.

---

Based on the replies on this question jQuery UI Dialog - missing close icon what I've I've tried so far without success:

1. loading bootstrap, and in fact loading it before jquery-ui-min
   that editing/adding this code into:

    // ==UserScript==
    // @name        test
    // @grant       GM_addStyle
    // @grant       GM_getResourceText
    // @require     http://code.jquery.com/jquery-2.1.4.min.js
    // @resource    source_1 https://maxcdn.bootstrapcdn.com/bootstrap/3.3.5/css/bootstrap.min.css
    // @require     http://code.jquery.com/ui/1.11.4/jquery-ui.min.js
    // @resource    source_2 http://code.jquery.com/ui/1.11.4/themes/smoothness/jquery-ui.css  
    // ==/UserScript==

    var src1 = GM_getResourceText ("source_1");
    GM_addStyle (src1);
    var src2 = GM_getResourceText ("source_2");
    GM_addStyle (src2);

    ...

    $(function() {
      $( "#dialog" ).dialog({
        title: 'My dialog'
      });
    });

and

2. via some dom manipulation on the Dialog Open event:
   that is, adding this extra code in the end:

   $(function() {           // or omitting this line (and the last one)
   $("#dialog").dialog({
       open: function() {
           $(this).closest(".ui-dialog")
           .find(".ui-dialog-titlebar-close")
           .removeClass("ui-dialog-titlebar-close")
           .html("<span class='ui-button-icon-primary ui-icon ui-icon-closethick'></span>");
       }
   });
   });    
   

---

How to fix this?