demo-watermark.css

body:after {                                                                                            
  content: 'DEMO';
  font-size: 319px;
  color: rgba(255,0,0,0.3);
  font-weight: 900;
  position: fixed;
  top: 50%;
  left: 0;
  right: 0;
  pointer-events: none;
  text-align: center;
}