Transform3d on mouse pointer in css3

I want to achieve the functionality at http://ift.tt/1Iw5NnF so i have created the css as on background hover

anim: hover { transform: translate3d(-21.6156px, 1.08132px, 20px);}

Now to change the value on hover it would need to get the pointer location so for that i have got

var x = event.clientX;
var y = event.clientY;
var coords =   x+5 , 5-y ;

Now the issue is how do i change the value from JavaScript to that particular CSS class.