title: Changing rgba background color on navbar based on scroll
body: <p>I've got a bootsrap fixed navbar at the top of my page. I'd like to gradually and smoothly increase the background-color opacity from 0 up to 0.75 based on the scroll position on the page. How can I do this using jquery?</p>
  
  <p>before scroll</p>
  
  <pre><code>.navbar-inverse {
    background: rgba(53,145,204,0);
  }
  </code></pre>
  
  <p>after scroll ~500px or better yet 100% of current window size</p>
  
  <pre><code>.navbar-inverse {
    background: rgba(53,145,204,0.75);
  }
  </code></pre>