JSFiddle - React, Tailwind, and code Playground

HTML

<div class="middle_center">
  <div class="blue_box"></div>
</div>

CSS

.middle_center {
  display: flex;
  align-items: center;
  justify-content: center;
  height: 400px;
}

.blue_box {
  width: 300px;
  height: 300px;
  background-color: #05c;
}