title: Bootstrap: set navbar-fixed-top background semi-transparent only when opened
body: <p>I'm building an Angularjs + Twitter Bootstrap application.<br>
  I would like to use a <em>navbar-fixed-top</em> class for my top navbar, because I want - in mobile (collapsed) mode - the drop-down of navbar items to be shown <em>over</em> the page contents, without pushing down contents.<br>
  I would also like to have the background of the <em>opened</em> drop-down to be semi-transparent.  I think I am quite there with a css like this:</p>
  
  <pre><code>.navbar-fixed-top {
    background-color: rgba(0,0,0,0.9);
    background-image: none;
  }
  </code></pre>
  
  <p>The only problem is that with this css the <em>closed</em> navbar is semi-transparent, too.
  Is it possible to set css attributes <em>only</em> to <em>opened</em> navbar?</p>
  
  <p><strong>UPDATE:</strong><br>
  From <em>srrvnn</em> answer (thanks!), I did solve with:</p>
  
  <pre><code>.navbar-collapse.collapsing,
  .navbar-collapse.in {
    position: absolute;
    width: 100%;
    background-color: rgba(0, 0, 0, 0.80);
  }
  </code></pre>