mutationObserverAPI looking for style changes

muteObserverStyle.js

// simple mutation observer
// listen to group of elements with target-class
// for style change and toggle class on root HTML

var modals = document.querySelectorAll('.target-class');
var rootHTML = document.getElementsByTagName( 'html' )[0];

var observer = new MutationObserver(function(mutations) {
    mutations.forEach(function(mutationRecord) {
        // do something on observed style change
        rootHTML.classList.toggle('.toggle-class');
    });    
});

modals.forEach(modal => {
    observer.observe(modal, { 
        attributes: true, 
        attributeFilter: ['style'] 
    });
});