body{
  margin: 0;
  padding: 0;
  min-height: 100vh;
  display: flex;
  justify-content:center;
  align-items:center;
  background: #000000;
  text-align: center; 
}

.box{
    position: relative;
    width: 800px;
    height: 600px;
    display: flex;
    justify-content:center;
    align-items:center;   
    background: rgba(0,0,0,1);  /* rgba wert kann auf 0,8 gestellt werden dann scheint der Border durch die box */
}    
 
.box:before{
content: '';
position: absolute;
    top: -2px;
    left: -2px;
    right: -2px;
    bottom: -2px;
    background: #fff;
    z-index: -1;
} 

.box:after{
content: '';
position: absolute;
    top: -2px;
    left: -2px;
    right: -2px;
    bottom: -2px;
    background: #fff;
    z-index: -2;
    filter: blur(40px);
}

.box:before, .box:after{
  background: linear-gradient(315deg,#89ff00,#060c21,#00bcd4);  
}

.content{
    padding: 20px;
    box-sizing: border-box;
    color: #FFFFFF;
    text-align: center;
}