(function() {
    var background = document.getElementById('background');

    window.addEventListener('mousemove', function (event) {
        var newX = (event.clientX / document.body.clientWidth * 2 - 1);
        var newY = (event.clientY / document.body.clientHeight * 2 - 1);
        background.style.top = -newY + "vh";
        background.style.left = -newX + "vw";
    })
})();