.main{
  height: 100%;
  width: 100%;
  background-image: linear-gradient(rgba(0, 0, 0, 0.4),rgba(0, 0, 0, 0.4)),url(banner.jpg);
  background-position: center;
  background-size: cover;
  position: absolute;
}
.form{
  width: 95%;
  height: auto;
  position: relative;
  margin: 20px 20px;
  background: #fff;
  padding: 5px;
  overflow: hidden;
}
/* Style buttons */
.btn {
  background-color: DodgerBlue;
  border: none;
  color: white;
  padding: 12px 30px;
  cursor: pointer;
  font-size: 20px;
}
.center {
  margin: 0;
  position: absolute;
  top: 50%;
  left: 50%;
  -ms-transform: translate(-50%, -50%);
  transform: translate(-50%, -50%);
}
/* Darker background on mouse-over */
.btn:hover {
  background-color: green;
}