An original version of this tip was contributed by ModeratorWes. As Zendesk updated the theming template, the code for the tip needed an upgrade.

The below code will work with the new theming template of the Zendesk theme. Let's get started.

Zendesk level:  Beginner
Knowledge:  HTML, CSS, JQUERY
Time Required:  10 minutes

A working example can be found here.

Follow the below steps to add and make it work:

1. Download the SVG file here.
2. Upload the svg file into Zendesk.  From your theme editor select “assets” and upload the arrow that you saved.
3. From your theme editor select “header.hbs”.  Add the following under the <header> tag.

<a href="#" class="cd-top">Top</a>

4. From your theme editor select “style.css”.  Add the following at the very top or very bottom.  Make sure you copy and paste your link from the assets in the background: selector.  You may also change the colour to match your theme on the background selector.

.cd-top {
  display: inline-block;
  height: 40px;
  width: 40px;
  position: fixed;
  bottom: 40px;
  right: 10px;
  box-shadow: 0 0 10px rgba(0, 0, 0, 0.05);
  /* image replacement properties */
  overflow: hidden;
  text-indent: 100%;
  white-space: nowrap;
  background: #F79420 url('$assets-cd-top-arrow-svg') no-repeat center 50%;
  visibility: hidden;
  opacity: 0;
  -webkit-transition: opacity .3s 0s, visibility 0s .3s;
  -moz-transition: opacity .3s 0s, visibility 0s .3s;
  transition: opacity .3s 0s, visibility 0s .3s;
}

.cd-top.cd-is-visible, .cd-top.cd-fade-out, .no-touch .cd-top:hover {
  -webkit-transition: opacity .3s 0s, visibility 0s 0s;
  -moz-transition: opacity .3s 0s, visibility 0s 0s;
  transition: opacity .3s 0s, visibility 0s 0s;
}

.cd-top.cd-is-visible {
  /* the button becomes visible */
  visibility: visible;
  opacity: 1;
}

.cd-top.cd-fade-out {
  /* if the user keeps scrolling down, the button is out of focus and becomes less visible */
  opacity: .5;
}

.no-touch .cd-top:hover {
  background-color: #F79420;
  opacity: 1;
}

@media only screen and (min-width: 768px) {
  .cd-top {
    right: 55px;
    bottom: 60px;
  }
}

@media only screen and (min-width: 1024px) {
  .cd-top {
    height: 30px;
    width: 30px;
    right: 55px;
    bottom: 60px;
  }
}

5. From your theme editor select “script.js”. Add the following under the document.addEventListener('DOMContentLoaded', function() {

// Up Arrow
// browser window scroll (in pixels) after which the "back to top" link is shown
var offset = 300,

// browser window scroll (in pixels) after which the "back to top" link opacity is reduced
offset_opacity = 1200,

//duration of the top scrolling animation (in ms)
scroll_top_duration = 700,

//grab the "back to top" link
$back_to_top = document.querySelector('.cd-top');

// hide or show the "back to top" link
window.addEventListener('scroll', function(e) {
  (window.scrollY > offset) ? $back_to_top.classList.add("cd-is-visible") : $back_to_top.classList.remove('cd-is-visible', 'cd-fade-out');
});

//smooth scroll to top
$back_to_top.addEventListener('click', function(e) {
  e.preventDefault();
  scroll({
   top: 0,
    behavior: "smooth"
  });
});

6. Save and Publish and when scrolling down the page you should see the new up arrow pop into view. Click on it and it should scroll you back to the top. 

Cheers.