Move hack from plugin file to functions.php

I have made a modification to the ‘wp-useronline’ plugin which simply adds a line at the top of my page. The change I have made is to a file called ‘template-tags.php’ and is as shown in the image below.

I would like to make this modification in ‘functions.php’ instead so that future updates do not impact it. Can someone please explain how I might accomplish this?

Thanks in advance…