button {
 background-color: #444141;
 border-radius: 4px;
 color: #fff;
 cursor: pointer;
 padding: 15px 30px;
 font-size: 18px;
 font-weight: bold;
 letter-spacing: 1px;
 border: none;
}

button:hover {
 background-image: linear-gradient(90deg, #53cbef 0%, #dcc66c 50%, #ffa3b6 75%, #53cbef 100%);
 animation: slidernbw 5s linear infinite;
 color: #000;
}

@keyframes slidernbw {
 to {
  background-position: 20vw;
 }
}