html {
  height: 100%;
  width: 100%;
}
.rainbow-button {
  display: inline-block;
  border: 2px solid;
  border-image-slice: 1;
  border-image-source: linear-gradient(to right, red, orange, yellow, green, blue, indigo, violet);
  padding: 10px 20px;
  color: white;
  text-transform: uppercase;
  font-size: 16px;
  font-weight: bold;
  letter-spacing: 1px;
  background-color: transparent;
  transition: background-color 0.3s ease;
}

.rainbow-button:hover {
  background-color: white;
  color: black;
}