class-change | manipulating CSS class names , triggering change events | Web Framework library

I'm trying to apply this patch to a file (bi_janrain_data/js/bi_janrain_data.js):

The WordPress theme I'm using adds two logos, a dark one when the site loads, and a lighter one when the sticky nav is activated upon scroll. When the sticky nav is active the class 'scrolled' is added to the containing div, 'scroll_header_top_area'.

The code below works if I've already scrolled down the page and the sticky nav is active and I refresh the page, but it doesn't switch out the logos dynamically when scrolling.

I've seen the suggestions to use MutationObserver but I'm afraid I can't understand the example or how to use it here.

How would I change this code to make the change dynamic based upon whether the 'scrolled' class is active.