CSS media Query

if screen width is less than 768px use this custom style:

#loginBody {
  width: 600px;
  height: 400px;
  margin-top: 100px;
  border-radius: 25px;
  background: rgb(238, 238, 238);
  background: -moz-linear-gradient(left, rgba(238, 238, 238, 1) 0%, #f8f8f8 100%);
  background: -webkit-linear-gradient(left, rgba(238, 238, 238, 1) 0%, #f8f8f8 100%);
  background: linear-gradient(to right, rgba(238, 238, 238, 1) 0%, #f8f8f8 100%);

  @media screen and (max-width: 768px) {
    width: 300px;
    height: 300px;
  }
}

http://www.w3schools.com/cssref/css3_pr_mediaquery.asp
http://www.w3schools.com/css/css_rwd_mediaqueries.asp
http://www.w3schools.com/css/css3_mediaqueries_ex.asp
http://www.w3schools.com/bootstrap/bootstrap_grid_system.asp