body {
  display: flex;
  justify-content: center;
  align-items: center;
  height: 100vh;
  margin: 0;
}
button {
  padding: 3rem;
  background-color: yellow;
  border: none;
  transition-duration: 600ms;
  transition-timing-function: ease;
  transition-properties: border-radius;
}
button:hover {
  border-radius: 2rem;
}
div {
  background-color: blue;
  position: fixed;
  top: 0;
  left: 0;
  width: 100%;
  height: 100%;
  z-index: -1;
  transition: background-color 600ms ease;
}
button:hover + div {
  background-color: springgreen;
}