Simple Lightbox for Gallery Section

Great theme BTW,

In the Gallery section of Onetone Options, i have the following code for eight thumbnails:

<div class="portfolio-list">
<ul>
	<li><a href="https://www.sennentimcke.com/wp-content/uploads/2014/04/16-copy.jpg"><img alt="" src="https://www.sennentimcke.com/wp-content/uploads/2014/04/16-copy.jpg" width="220" height="220" /></a></li>
	<li><a href="https://www.sennentimcke.com/wp-content/uploads/2014/04/1-copy.jpg"><img alt="" src="https://www.sennentimcke.com/wp-content/uploads/2014/04/1-copy.jpg" width="220" height="220" /></a></li>
</ul>
</div>

(etc for the remaining images)

This enlarges each image within the current window and the user then needs to click back to the homepage. Is there a way to implement a lightbox feature here? I am new to such customisations and have got lost reading up on ways to do this effectively..

Any support would be greatly appreciated!

As seen here:
https://www.sennentimcke.com