title: CSS Opacity for a button
body: <p>I have a button in HTML and I have to add opacity to it. I want to use CSS to do that. Here is my existing CSS code for button:</p>
  
  <pre><code>.btn_nav{
  position:fixed;
  width:100%;
  height:68px;
  background-color:#323232;
  border-bottom:2px solid #777777;
  }
  </code></pre>