JSFiddle - React, Tailwind, and code Playground

HTML

<div class="box"></div>

CSS

html {
  height: 100%;
}

body {
  min-height: 100%;
  background: url('http://www.planwallpaper.com/static/images/1968081.jpg') center/cover no-repeat;
}

.box {
  width: 300px;
  height: 300px;
  background: navy;
  opacity: 0.4;
}