MyBB YouTube Video Iframe Class, where to add css class?

Question

I wrapped the iframe with the below <div> classes, but no matter where I put the CSS, it doesn't pick up the CSS,it just isn't working

HTML:

<div class="auto-resizable-iframe">
  <div>
    <iframe frameborder="0" allowfullscreen="" src="//www.youtube.com/embed/{$id}"></iframe>
  </div>
</div>

And here is the CSS, I tried putting it in global.css, and every other css area, but nothing....

.auto-resizable-iframe {
  max-width: 820px;
  margin: 0px auto;
}

.auto-resizable-iframe > div {
  position: relative;
  padding-bottom: 75%;
  height: 0px;
}

.auto-resizable-iframe iframe {
  position: absolute;
  top: 0px;
  left: 0px;
  width: 100%;
  height: 100%;
}