.image-box {
  /* Here's the trick */
  background: linear-gradient(rgba(0,0,0,0.5), rgba(0,0,0,0.5)), var(--image-url) center center;

  /* Here's the same styles we applied to our content-div earlier */
  color: white;
  min-height: 100vh;
  display: flex;
  align-items: center;
  justify-content: center;
}

/* For demo only */
html, body {
  margin: 0;
  padding: 0;
  font-family: sans-serif;
}
h1 {
  font-size: 40px;
}