$(document).mousemove(function(event) {
windowWidth = $(window).width();
windowHeight = $(window).height();
mouseXpercentage = Math.round(event.pageX / windowWidth * 100);
mouseYpercentage = Math.round(event.pageY / windowHeight * 100);
$('.radial-gradient').css('background', 'radial-gradient(at ' + mouseXpercentage + '% ' + mouseYpercentage + '%, #3498db, #9b59b6)');
});