title: Button doesn't work and change the border color
body: <p>I tried to code a little Firefox Extension but I don't get the Button in the popup working.
  What should happen when you click the button is that the red border changes into a green one.
  I couldn't find the mistake.</p>
  <p>This is my manifest.json</p>
  <pre><code>{
  
    &quot;manifest_version&quot;: 2,
    &quot;name&quot;: &quot;Testing&quot;,
    &quot;version&quot;: &quot;1.0&quot;,
  
    &quot;description&quot;: &quot;Testing&quot;,
  
    &quot;icons&quot;: {
      &quot;48&quot;: &quot;icon.jpg&quot;
    },
  
    &quot;content_scripts&quot;: [
      {
        &quot;matches&quot;: [&quot;*://*.allee-abi20.de/*&quot;],
        &quot;js&quot;: [&quot;ust.js&quot;]
      }
    ],
      
    &quot;permissions&quot;: [
      &quot;activeTab&quot;,
      &quot;contextMenus&quot;
    ],
    
    &quot;browser_action&quot;: {
          &quot;default_icon&quot;: &quot;icon.jpg&quot;,
          &quot;default title&quot;: &quot;Hallo&quot;,
          &quot;default_popup&quot;: &quot;view.html&quot;
      }
  }
  </code></pre>
  <p>Thats the view.html</p>
  <pre><code>&lt;!DOCTYPE html&gt;
  &lt;html&gt;
      &lt;head&gt;
          &lt;meta charset=&quot;utf-8&quot;/&gt;
          &lt;link rel=&quot;stylesheet&quot; href = &quot;style.css&quot; type=&quot;text/css&quot;/&gt; 
      &lt;/head&gt;
      &lt;body&gt;
          &lt;input type=text id=&quot;Kat&quot;&gt;&lt;/input&gt;
          &lt;input type=text id=&quot;Ty&quot;&gt;&lt;/input&gt;
          &lt;button id= &quot;but&quot;&gt;Klicken &lt;/button&gt;     
      &lt;script type=&quot;text/javascript&quot; src=&quot;./popup.js&quot;&gt;&lt;/script&gt;   
      &lt;/body&gt;
  &lt;/html&gt;
  </code></pre>
  <p>And thats the popup.js</p>
  <pre><code>document.body.style.border = &quot;5px solid red&quot;;
  var myLink = document.getElementById(&quot;But&quot;);
  myLink.onclick = function(){
      document.body.style.border = &quot;20px solid green&quot;;
  }
  </code></pre>