You could bring that to your theme if you want to. As for "is it difficult" always will have a relative answer, since it depends on how much experience you have (with both HTML and concrete5). You could also make it as an Add-On, but that probably will be too much for something simple (at least, if you don't really configure anything that is).
- Extract "themes" directory and "pace.js" from the download (on the PACE website) into that directory
- Open up /elements/header_top.php (or header.php) of your theme
- Add this code before "</head>":
<script src="<?php echo $view->getThemePath()?>/pace/pace.js"></script>
<link href="<?php echo $view->getThemePath()?>/pace/themes/pace-theme-barber-shop.css" rel="stylesheet" />
This will use the "Barber Shop" theme. You can change the .css file to point to another theme if you don't like it. It depends what theme you are using where your theme is located.
Here are some docs about concrete5 theming:
The problem is that the animation will remain on the screen permanently. Eg, animation "big-counter" jammed in 99% ? Something goes wrong..