<div class="radial-gradient"></div>

index.html

.radial-gradient {
    position:fixed;
    top:0px;
    left:0px;
    height:100%;
    width:100%;

    /*Fallback if gradeints don't work */
    background: #ff0000;
    /*Linear gradient... */
    background:
      radial-gradient(
       at center, #ff0000, #0000ff
      );
  }

index.css

$(document).mousemove(function(event) {

    windowWidth = $(window).width(); // vw
    windowHeight = $(window).height(); // vh

    mouseXpercentage = Math.round(event.pageX / windowWidth * 100);
    mouseYpercentage = Math.round(event.pageY / windowHeight * 100);

    $('.radial-gradient').css('background', 'radial-gradient(at ' + mouseXpercentage + '% ' + mouseYpercentage + '%, #ff0000, #0000ff)');

  });

index.js