style.css

* {
  margin: 0;
  padding: 0;
  box-sizing: border-box;
}

.popup {
  position: fixed;
  top: 0;
  bottom: 0;
  left: 0;
  right: 0;
  z-index: 20;
  background-color: rgba(0,0,0,.7);
  padding: 15px;
  overflow: auto;
  text-align: center;
  font-size: 0;
  visibility: hidden;
  opacity: 0;
}

.popup--active {
  visibility: visible;
  opacity: 1;
}

.popup__inner, .popup::before {
  display: inline-block;
  vertical-align: middle;
}

.popup::before {
  content: "";
  height: 100%;
  width: 0;
}

.popup__inner {
  background-color: #fff;
  width: 100%;
  max-width: 450px;
  font-size: 16px;
  padding: 50px;
  position: relative;
}

.popup__close {
  display: inline-block;
  width: 20px;
  height: 20px;
  position: absolute;
  top: 15px;
  right: 15px;
  z-index: 1;
}

.btn {
  display: inline-block;
  background-color: #000;
  color: #fff;
  min-width: 200px;
  text-transform: uppercase;
  padding: .5em;
  text-align: center;
}