How to use border-radius in slimscroll

$(document).ready(function(e) { 
  $('.slimmscroll').slimscroll({
    color: 'rgba(0,0,0,0.5)',
    size: '4px',
    radius : '40px',
    height: '100px',
    alwaysVisible: true
  });
});

<script src="https://raw.github.com/rochal/jQuery-slimScroll/master/jquery.slimscroll.min.js"></script>
<script src="https://ajax.googleapis.com/ajax/libs/jquery/2.0.0/jquery.min.js"></script>

<div class="slimmscroll">
  <h1>Simple Sidebar</h1>
  <p>This template has a responsive menu toggling system. The menu will appear collapsed on smaller screens, and will appear non-collapsed on larger screens. When toggled using the button below, the menu will appear/disappear. On small screens, the page content will be pushed off canvas.</p>
  <p>Make sure to keep all page content within the <code>#page-content-wrapper</code>.</p>
 <p>This template has a responsive menu toggling system. The menu will appear collapsed on smaller screens, and will appear non-collapsed on larger screens. When toggled using the button below, the menu will appear/disappear. On small screens, the page content will be pushed off canvas.</p>
  <p>Make sure to keep all page content within the <code>#page-content-wrapper</code>.</p>

</div>