html {
    height: 100%;
}


body {
    height: 100%;
    margin: 0;
    background: linear-gradient(to top,  #FCFCFC 0%,#F1F1F1 100%); /* W3C */
}


#login_form {
   position:absolute;
   top:50%;
   left:50%;
   width:220px;
   height:160px;
   margin-left:-110px; /* largeur/2 */
   margin-top:-80px; /* hauteur/2 */
}