如何在网页向下滚动时添加CSS属性？

window.addEventListener('scroll', function() {
  var scrolled = window.scrollY;
  
  if (scrolled > 100) {
    document.body.classList.add('scrolled');
  } else {
    document.body.classList.remove('scrolled');
  }
});

body.scrolled {
  background-color: #f0f0f0;
}