<div class="layout">
  <div class="popup">
    <p>Hello</p>
    <p>World</p>
  </div>
</div>

.layout {
  display: flex;
  align-items: center;
  justify-content: center;
  min-height: 100vh;
}

.popup {
  background: green;
  width: 100px;
  height: 100px;
 }

body {
  margin: 0;
}