style.css

@import url('https://fonts.googleapis.com/css?family=Roboto&display=swap');

* {
  box-sizing: border-box;
}

body {
  margin: 0;
  min-height: 100vh;
  background: linear-gradient(-45deg, #ee7752, #e73c7e, #23a6d5, #23d5ab);
  background-size: 400% 400%;
  animation: gradient 15s ease infinite;
  height: 100vh;
  color: #000;
  font-family: 'Roboto', sans-serif;
  font-weight: 300;
  text-align: center;
  display: flex;
  align-items: center;
  justify-content: center;
}
@keyframes gradient {
  0% {
    background-position: 0% 50%;
  }
  50% {
    background-position: 100% 50%;
  }
  100% {
    background-position: 0% 50%;
  }
}

.button-container {
  border: 1px solid rgba(0, 0, 0, 0.5);
  padding: 0.5rem;
  border-radius: 1rem;
  box-shadow: inset 0 1.25rem 0.25rem -1.2rem rgba(255, 255, 255, 0.7);
}

#btn {
  cursor: pointer;
  outline: none;
  width: 8.75rem;
  height: 4.25rem;
  font-family: 'Roboto', sans-serif;
  font-style: bold;
  font-size: 1rem;
  color: rgb(255, 255, 255);
  text-shadow: 0.1px 0.1px 0.1px black;
  background: linear-gradient(to top, #696969, #575757);
  border: 2px solid black;
  border-radius: 7px;
  box-shadow: inset 0 20px 4px -19px rgba(255, 255, 255, 0.4), 0 12px 12px 0 rgba(0, 0, 0, 0.3);
}

#btn:hover {
  background: linear-gradient(to bottom, #c7c7c7, #444444);
}

#btn:active {
  transform: translateY(3px);
  box-shadow: 0 6px 6px 0 rgba(0, 0, 0, 0.3);
}