title: extjs6 theming guide - mixing not being recognized
body: <p>I am following the ExtJS6 theming guide and creating my first mixin on a panel.
  I've created a theme in packages/local/my-classic-theme2/sass/var/panel/Panel.scss
  The panel is not being changed when I run and I am getting the following errors. in the panel.scss it says undeclared mixin.</p>
  
  <p>sencha app watch error in picture<a href="https://i.stack.imgur.com/wQASb.jpg" rel="nofollow noreferrer"><img src="https://i.stack.imgur.com/wQASb.jpg" alt="enter image description here"></a></p>
  
  <p>am I missing a step?</p>
  
  <pre><code>@include extjs-panel-ui(
      $ui: 'highlight-framed',
      $ui-header-background-color: red,
      $ui-border-color: red,
      $ui-header-border-color: red,
      $ui-body-border-color: red,
      $ui-border-width: 5px,
      $ui-border-radius: 5px,
      $ui-header-color: white
  );
  </code></pre>
  
  <p>and I put this on the actual panel</p>
  
  <pre><code>ui: 'highlight',
  frame: true,
  </code></pre>