title: use css variable with scss darken and ligthen filter
body: <p>I would like to use scss <code>lighten()</code> and <code>darken()</code> function with CSS4 <code>var</code>.</p>
  
  <p>Like this : </p>
  
  <pre><code>box-shadow: inset 0px -2px 50px 45px darken(var(--colorbtn),20%);
  </code></pre>
  
  <p>It seems not working, gulp write this:</p>
  
  <pre><code>Error: $color: "var(--colorbtn)" is not a color for `darken'
  </code></pre>
  
  <p>Its not possible to do that?</p>