body{
  margin: 0;
  padding: 0;
  background: rgb(47, 52, 64)
}
.btn{
  text-decoration: none;
  position: absolute;
  top: 50%;
  left: 50%;
  transform: translate(-50%,-50%);
  font-size: 24px;
  font-family: "STKaiTi";
  background: linear-gradient(90deg,#03a9f4,#f441a5,#ffeb3b,#03a9f4);
  background-size: 400%;
  width: 400px;
  height: 100px;
  line-height: 100px;
  text-align: center;
  text-transform: uppercase;
  color: white;
  border-radius: 50px;
  z-index: 1;
  animation: sun 8s linear infinite;
}
.btn::after{
  content: "";
  position: absolute;
  left: -5px;
  right: -5px;
  top: -5px;
  bottom: -5px;
  background: linear-gradient(90deg,#03a9f4,#f441a5,#ffeb3b,#03a9f4);
  background-size: 400%;
  border-radius: 50px;
  filter: blur(20px);
  z-index: -1;
  animation: sun 8s linear infinite;
}
.btn:hover::before{

}
.btn:hover{

}

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