body {
  position: relative;
  z-index: 1;
  color: white; /* example content color */
}

body::before {
  content: "";
  position: absolute;
  top: 0;
  left: 0;
  right: 0;
  bottom: 0;
  background-size: cover;
  background-position: center;
  background-repeat: no-repeat;
  opacity: 0.5; /* adjust to your desired opacity */
  z-index: -1;
}