Allows you to easily apply styles based on current Y scroll value. It does this by defining a breakpoint (yes, a single one) and adding/removing a class as you cross that. Implemented with setInterval so it's resource-efficient and won't overwhelm your CPU when scrolling slowly. Has no dependencies, simple as hell.

scrollBeyond.js

setTimeout(function() {

    var amount          = 100;                  //  how many pixels before the miracle happens
    var beyondClass     = 'beyond-that-point';  //  this class will be added
    var targetSelector  = 'body';               //  which element to add the class to
    var checkMS         = 20;                   //  check scroll position every N milliseconds

    var eClass = document.querySelector(targetSelector).classList;
    setInterval(function() {
        var y = window.scrollY;
        var c = beyondClass;
        var isBeyond = !!(y>=amount)?1:0;
        if(isBeyond==1) if(!eClass.contains(c)) eClass.add(c);
        if(isBeyond==0) if( eClass.contains(c)) eClass.remove(c);
    },checkMS);

},1);