.shadow {
  position: relative;
  /*margin: 200px auto 0;*/
  /*width: 400px;*/
  /*height: 250px;*/
  /*background: linear-gradient(0deg,#000,#262626);*/
}

.shadow:before, .shadow:after {
  content: '';
  position: absolute;
  /*top: -2px;*/
  /*left: -2px;*/
  background: linear-gradient(45deg,#fb0094,#0000ff,#00ff00,#ffff00,#ff0000,#fb0094,#0000ff,#00ff00,#ffff00,#ff0000);
  /*background-size: 400%;*/
  width: calc(95%);
  height: calc(95%);
  z-index: -10;
  animation: animate 20s linear infinite;
}

.shadow:after {
  filter: blur(35px);
}