title: Inverting colours of an image callback in fullpage.js
body: <p>I've changed the slide arrows to my own png, black arrows but on some slides, which have darker background I want to invert them so they are white. </p>
  
  <pre><code>-webkit-filter: invert(100%);
  </code></pre>
  
  <p>works just fine, but how to trigger this only on slides that using a callback? </p>
  
  <p>Quick animation of the invert parameter 0% > 100% would be a cool bonus. Thanks.</p>