transition

<a class="btn btn1">ボタン</a>
<a class="btn btn2">ボタン</a>

.btn {
  width: fit-content;
  padding: 20px 60px;
  border: 1px solid #000;
  color: #fff;
  background-color: #000;
  display: inline-block;
}

.btn:hover {
  border: 1px solid #fff;
  color: #000;
  background-color: #fff;
}