body {
  display: flex;
  justify-content: center;
  align-items: center;
  min-height: 100vh;
}
button {
  background: green;
  color: white;
  width: 200px;
  height: 200px;
  border-radius: 50%;
  font-size: 3rem;
  border: 0;
  box-shadow: 0px 0px 0px 3px black;
  transition: .3s;
}

button:focus,
button:hover {
  box-shadow: 0px 0px 0px 5px black;
}