body.login_admin:before {
    content: " ";
    background: rgba(31, 31, 31, 0.7);
    min-height: 100%;
    min-width: 100%;
    position: absolute;
}
body.login_admin {
  background: url('../../assets/images/bg-login.jpg') no-repeat center center fixed; 
  -webkit-background-size: cover;
  -moz-background-size: cover;
  -o-background-size: cover;
  background-size: cover;
}