title: Modify alpha opacity of LESS variable
body: <p>Using LESS, I know that I can change the saturation or tint of a color variable. That looks like this:</p>
  
  <pre><code>background: lighten(@blue, 20%);
  </code></pre>
  
  <p>I want to change the alpha opacity of my color, though. Preferably like this:</p>
  
  <pre><code>background: alpha(@blue, 20%);
  </code></pre>
  
  <p>Is there a simple way to do this in LESS? </p>