style.css

* {
  margin: 0;
  padding: 0;
  box-sizing: border-box;
}

body {
  height: 100vh;
  background: #131416;
  display: flex;
  justify-content: center;
  align-items: center;
}

.box {
  width: 400px;
  height: 300px;
  background: rgb(31, 31, 31);
  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%;
  background-size: 500%;
  z-index: -1;
  transform: scale(1.02);
  animation: animate 20s infinite;
}

.box:after { 
  filter: blur(20px);
  }

@keyframes animate { 
  0% { background-position: 0 0; }
  50% { background-position: 300% 0; }
  100% { background-position: }
  }