make sticky header

function stickyHeader() {
  window.onscroll = function() {
    const header = document.getElementById('header');
    const sticky = header.offsetTop;
    
    if (window.pageYOffset > sticky) {
      header.classList.add('sticky');
      header.classList.add('container');
    } else {
      header.classList.remove('sticky');
      header.classList.remove('container');
    }
  };
}