Background responsive

Hi,

How can i edit code of background ?
I would like insert this code :
background:url(…/images/mybackground.jpg) no-repeat center top;
-webkit-background-size: cover;
-moz-background-size: cover;
-ms-background-size: cover;
-o-background-size: cover;
background-size: cover;

I checked files.css in folder css but i think, i missed code of background.

My web site adress :
http://theunitofairsoft.fr/
I would like my background responsive 🙂

Best Regards,