They have applied a custom CSS for the form.If you go to their demo site and click on the Open Form under Popup with form and inspect, you will see .white-popup-block has styles which is not included in the default CSS. You need to apply those styles to the container.

.white-popup-block {
  background: #FFF;
  padding: 20px 30px;
  text-align: left;
  max-width: 650px;
  margin: 40px auto;
  position: relative;
}

Fiddle