.radial-gradient {
  position:absolute;
  opacity:0.5;
  z-index:100;
  top:50%;left:50%;
  position:absolute;
  width:780px;height:580px;
    margin-left:-390px;
    margin-top:-290px;
}

    .cadre {
    top:50%;left:50%;
    position:absolute;
    width:800px;height:600px;
    margin-left:-400px;
    margin-top:-300px;
    object-fit: scale-down;
    background-image: url(cadre.png);
    background-position:center center;
    background-repeat: no-repeat;
    background-size: cover;  
    }

/*  background: #9b59b6;
  Linear gradient... 
  background: 
    radial-gradient(
     at center, #3498db, #9b59b6*/
    );
}