JavaScript, having trouble changing an elements' style
<p>So this is my index file:</p>

<pre><code>&lt;!DOCTYPE html&gt;
&lt;html lang="en"&gt;
    &lt;head&gt;
        &lt;meta http-equiv="X-UA-Compatible" content="IE=edge" /&gt;
        &lt;meta http-equiv="pragma" content="no-cache"/&gt;
        &lt;meta name="apple-mobile-web-app-capable" content="yes" /&gt;
        &lt;meta name ="viewport" content="width=device-width, initial-scale=1.0, maximum-scale=1.0, user-scalable=0" /&gt;
        &lt;meta name="apple-mobile-web-app-status-bar-style" content="black-translucent" /&gt;
        &lt;meta charset="utf-8"/&gt;

        &lt;title&gt;TEST&lt;/title&gt;

        &lt;style&gt;
            body {
              background: #000000;
              color:#cccccc;
              margin: 0px;
              padding: 0px;
              border: 0px;
            }
            canvas {
                      image-rendering: optimizeSpeed;
                      -webkit-interpolation-mode: nearest-neighbor;
                      -ms-touch-action: none;
                      margin: 0px;
                      padding: 0px;
                      border: 0px;
            }
            :-webkit-full-screen #canvas {
                 width: 100%;
                 height: 100%;
            }
            div.gm4html5_div_class
            {
              margin: 0px;
              padding: 0px;
              border: 0px;
            }

            div.gm4html5_login
            {
                 padding: 20px;
                 position: absolute;
                 border: solid 2px #000000;
                 background-color: #404040;
                 color:#00ff00;
                 border-radius: 15px;
                 box-shadow: #101010 20px 20px 40px;
            }
            div.gm4html5_cancel_button
            {
                 float: right;
            }
            div.gm4html5_login_button
            {
                 float: left;
            }
            div.gm4html5_login_header
            {
                 text-align: center;
            }

            :-webkit-full-screen {
               width: 100%;
               height: 100%;
            }
        &lt;/style&gt;
    &lt;/head&gt;

    &lt;body&gt;
        &lt;div class="gm4html5_div_class" id="gm4html5_div_id"&gt;
            &lt;canvas id="canvas" width="320" height="480"&gt;
               &lt;p&gt;Your browser doesn't support HTML5 canvas.&lt;/p&gt;
            &lt;/canvas&gt;
        &lt;/div&gt;

        &lt;script type="text/javascript" src="html5game/TEST.js?GPJZB=444106034"&gt;&lt;/script&gt;
    &lt;/body&gt;
&lt;/html&gt;
</code></pre>

<p>I can't seem to change the style of <code>div.gm4html5_div_class</code> using js. I've tried a bunch of things, but no luck.</p>

<p>Can anyone please help me with this?
Also note that it might be because I don't know enough about js...</p>