style.css

body {
  background-color: #000;
  margin: 0;
  padding: 0;
  color: white;
}

h1 {
  text-align: center;
  margin: 0;
  background-color: tomato;
  padding: 10px;
}


.shadow {
  position: relative;
  width: 400px;
  height: 250px;
  background: linear-gradient(#111, #333);
  margin: 100px auto auto;
  padding: 10px 30px;
  line-height: 2;
  text-align: center;
}
.shadow .h1{
  background: linear-gradient(50deg,red,green,yellow,lime,purple);
  -webkit-background-clip: text;
  -webkit-text-fill-color: transparent;
  text-shadow: 0 0 3px lime;
}

.shadow p{
  background: linear-gradient(50deg,red,green,yellow,lime,purple);
  -webkit-background-clip: text;
  -webkit-text-fill-color: transparent;
}
.shadow:before, .shadow:after {
  content: '';
  position: absolute;
  top: -2px;
  left: -2px;
  width: calc(100% + 4px);
  height: calc(100% + 4px);
  background: linear-gradient(50deg, red, green, blue, yellow, lime, purple);
  background-size: 400%;
  z-index: -1;
  animation: animate 20s linear infinite;
}
.shadow:after {
  filter: blur(10px);
}

@keyframes animate {
  0% {
    background-position: 0 0;
  }
  50% {
    background-position: 400%;
  }
  100% {
    background-position: 0 0;
  }
}