title: Change CSS class content from javascript [Firefox/XUL]
body: <p>I'm working on a firefox addon, and relies heavily on some dynamic changes with javascript. There are some UI elements (hbox/descriptions), which are styled using CSS classes. Now I wish to change the  a CSS class defintion from javascript, without having to work element id's. Is it possible ?</p>
  
  <p>Similar questions had been asked before, but most of them are using php to generate a CSS or jquery based changes. 
  Here's an example for my scenario: 
  My XUL : </p>
  
  <pre><code>&lt;box id="firefox-addon-box"&gt;
    &lt;description id="description-1" class="description-text"&gt; Some description &lt;/description&gt;
    &lt;description id="description-2" class="description-text"&gt; Some description &lt;/description&gt;
  &lt;/box&gt;
  </code></pre>
  
  <p>My CSS : </p>
  
  <pre><code>.description-text {
    font-size: 15px; 
    color: #fff; 
  }
  </code></pre>
  
  <p>So I wish to change the font-size in <code>description-text</code> class to say, 20px dynamically. 
  Now I know that we can <a href="https://developer.mozilla.org/en-US/docs/Web/Guide/CSS/Getting_started/JavaScript#Action%3a_A_JavaScript_demonstration" rel="nofollow">change the style using individual</a> id's, with a <code>getElementById()</code> and changing the style. But here I wish to change a class (which matters to a lot of elements), rather than to work on individual ids.
  I also know that <a href="http://books.mozdev.org/html/mozilla-chp-5-sect-3.html#mozilla-CHP-5-SECT-3.3" rel="nofollow">we can point to another class definition</a> by <code>setAttribute()</code> and change 'class' - this would again require working on individual id's. In reality, I have plenty of UI elements (all pointing to one CSS class), and many values changed in a CSS class. So my question is, is there a way to directly change a CSS class definition from javascript, without having to work on element ids ?
  Thanks for any help.</p>