Rainbow button

rainbow-button.css

html, body {
  height: 100%;
  overflow: hidden;
}
body {
  background: #191919;
  display: flex;
  align-items: center;
  justify-content: center;
}
.button {
  background-image: linear-gradient(90deg, #00C0FF 0%, #FFCF00 49%, #FC4F4F 100%);
  position: relative;
  padding: 3px;
  display: inline-block;
  border-radius: 17px;
}
.button span {
  display: inline-block;
  background: #191919;
  color: white;
  text-transform: uppercase;
  padding: 2rem 5rem;
  border-radius: 15px;
  font-family: "arial";
  font-size: 3rem;
  font-weight: 800;
}

/*
<a href="#" class="button">
  <span>Button</span>
</a>
*/