<div class="stage">
  <button class="btn">Hover me</button>
</div>

.stage {
  box-sizing: border-box;
  position: absolute;
  left: 0;
  top: 0;
  width: 100vw;
  height: 100vh;
  overflow: hidden;
  display: flex;
  align-items: center;
  justify-content: center;
  background-color: #65799b;
}

.btn {
  width: 200px;
  font-size: 20px;
  height: 60px;
  border: none;
  outline: none;
  background: #c38b8b;
  color: #fff;
  cursor: pointer;
  position: relative;
  z-index: 0;
  border-radius: 10px;
}

.btn::before {
  position: absolute;
  content: "";
  width: calc(100% + 4px);
  height: calc(100% + 4px);
  left: -2px;
  top: -2px;
  background: linear-gradient(
    124deg,
    #ff2400,
    #e81d1d,
    #e8b71d,
    #e3e81d,
    #1de840,
    #1ddde8,
    #2b1de8,
    #dd00f3,
    #dd00f3
  );
  background-size: 400%;
  z-index: -1;
  filter: blur(5px);
  animation: move 20s linear infinite;
  opacity: 0;
  transition: opacity 0.3s ease-in-out;
  border-radius: 10px;
}

.btn:hover::before {
  opacity: 1;
}

.btn::after {
  z-index: -1;
  content: "";
  position: absolute;
  width: 100%;
  height: 100%;
  background: #555273;
  left: 0;
  top: 0;
  border-radius: 10px;
}

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