loader.css

.loader {
  border: 16px solid #f3f3f3;
  border-radius: 50%;
  border-top: 16px solid blue;
  width: 120px;
  height: 120px;
  -webkit-animation: spin 2s linear infinite;
  animation: spin 2s linear infinite;
}

.center {
  position: absolute;
  top: 50%;
  left: 50%;
  transform: translate(-50%,-50%);
  -ms-transform: translate(-50%,-50%);
}

@-webkit-keyframes spin {
  0% { -webkit-transform: rotate(0deg); }
  100% { -webkit-transform: rotate(360deg); }
}

@keyframes spin {
  0% { transform: rotate(0deg); }
  100% { transform: rotate(360deg); }
}

#overlay {
  position: fixed; /* Sit on top of the page content */
  display: none; /* Hidden by default */
  width: 100%; /* Full width (cover the whole page) */
  height: 100%; /* Full height (cover the whole page) */
  top: 0;
  left: 0;
  right: 0;
  bottom: 0;
  background-color: rgba(0,0,0,0.5); /* Black background with opacity */
  z-index: 2; /* Specify a stack order in case you're using a different order for other elements */
  cursor: pointer; /* Add a pointer on hover */
}