:hover

.joinBtn {
  width: 10em;
  height: 5ex;
  background-color: gold;
  border: 2px solid firebrick;
  border-radius: 10px;
  font-weight: bold;
  color: black;
  cursor: pointer;
}

.joinBtn:hover {
  background-color: bisque;
}

<p>Would you like to join our quest?</p>
<button class="joinBtn">Confirm</button>

:hover {
  /* ... */
}

<a href="#">Try hovering over this link.</a>

a {
  background-color: powderblue;
  transition: background-color 0.5s;
}

a:hover {
  background-color: gold;
}