title: Use a mixins to change variable value
body: <p>I am generally quite new to sass and just wondered if it possible to define a new variable value, using mixins?</p>
  
  <p>I thought it would be good to use a variable to define the padding size throughout the website I am building. The idea being that I could then use a mixin and media queries to define a new value.</p>
  
  <p>An example might make more sense:</p>
  
  <pre><code>$site-padding:  40px;
  
  @include media-over-767 {
      $site-padding: 20px;
  }
  </code></pre>
  
  <p>I think this would be super useful but cannot get it to work... is there a method that works?</p>