.box 
    {
  width: 400px;
  height: 300px;
  /* background-color: rgb(15, 61, 55); */
  position: relative;
    }

.box::before,
.box::after 
{
  content: '';
  position: absolute;
  top: 0;
  left: 0;
  background: linear-gradient(45deg, #ff0000, #00f0f0, #00ff00, #0000ff, #ff0000, #00f0f0, #00ff00, #0000ff, #f00f0f);
  width: 100%;
  height: 100%;
  transform: scale(1.02);
  z-index: -1;
  background-size: 500%;
  animation: animate 20s infinite;
}


.box::after 
{
  filter: blur(20px);
}
@keyframes animate {
  0% { background-position: 0 0; }
  50% { background-position: 300% 0; }
  100% { background-position: 0 0; }
}