Preview:
<!DOCTYPE html>
<html>
<head>
<link rel="stylesheet" href="https://cdnjs.cloudflare.com/ajax/libs/font-awesome/4.7.0/css/font-awesome.min.css">
<style>
body {
  font-family: Arial;
}
​
* {
  box-sizing: border-box;
}
​
.openBtn {
  background: #f1f1f1;
  border: none;
  padding: 10px 15px;
  font-size: 20px;
  cursor: pointer;
}
​
.openBtn:hover {
  background: #bbb;
}
​
.overlay {
  height: 100%;
  width: 100%;
  display: none;
  position: fixed;
  z-index: 1;
  top: 0;
  left: 0;
  background-color: rgb(0,0,0);
  background-color: rgba(0,0,0, 0.9);
}
​
.overlay-content {
  position: relative;
  top: 46%;
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