@mixin popup($width, $height: 200px) {
  position: fixed;
  top: 50%;
  left: 50%;
  width: $width;
  margin-top: -$height/2;
  margin-left: -$width/2;
  padding: 30px;
  background-color: #DDD;
  text-align: center;
  font-family: Arial, Helvetica, sans-serif;
  z-index: 200;
  border-radius: 20px;
  box-shadow: 0 0 1em #888;
}