I've CSS defined to add watermark in the page. with the code below :

body:before{
  content: '';
  position: fixed;
  top: 0;
  bottom: 0;
  left: 0;
  right: 0;
  z-index: -1;
  
  color: #0d745e;
  font-size: 100px;
  font-weight: 500px;
  display: grid;
  justify-content: center;
  align-content: center;
  opacity: 0.2;
  transform: rotate(-45deg);
}

what I need to do is create a JavaScript function to edit the content properties to enter a value. how to achieve that. Thanks