<a href="#0" class="button">
  <span>Button</span>
</a>

.button {
  background-image: linear-gradient(90deg, #00C0FF 0%, #FFCF00 49%, #FC4F4F 100%);
  position: relative;
  padding: 3px;
  display: inline-block;
  border-radius: 7px;
  span {
    display: inline-block;
    background: #191919;
    color: white;
    text-transform: uppercase;
    padding: 2rem 5rem;
    border-radius: 5px;
    font-family: -apple-system, BlinkMacSystemFont, "Segoe UI", Roboto, Helvetica, Arial, sans-serif, "Apple Color Emoji", "Segoe UI Emoji", "Segoe UI Symbol";
    font-weight: 800;
    font-size: 3rem;
  }
}

html, body {
  height: 100%;
  overflow: hidden;
}
body {
  background: #191919;
  display: flex;
  align-items: center;
  justify-content: center;
}