style.css

body {
  background-image: linear-gradient(#16222a, #3a6073);
}

#container {
  background-image: linear-gradient(45deg, #bdc3c7, #2c3e50, #bdc3c7, #2c3e50);
  width: 400px;
  margin: 30px auto 0;
  padding: 40px;
  box-sizing: border-box;
  box-shadow: 15px 15px 15px -8px #000;
  border-radius: 10px;
  text-align: center;
}

h1 {
  font-family: "Poppins", sans-serif;
  margin: 0 0 20px;
  padding: 0 0 20px;
  border-bottom: solid 1px #ddd;
}

form {
  font-family: "Poppins", sans-serif;
  text-align: left;
}

form label {
  display: block;
  margin: 25px 0;
  font-weight: bold;
}

form input,
for select {
  padding: 8px;
  margin: 10px 0;
}

form input[type="text"] {
  width: 90px;
}

button {
  background-color: #20232b;
  color: white;
  border: none;
  border-bottom: solid 4px rgb(53, 46, 46);
  text-transform: uppercase;
  font-size: 18px;
  font-family: "Poppins", sans-serif;
  padding: 20px 30px;
  width: 100%;
  border-radius: 10px;
}

button:hover {
  background-image: linear-gradient(#0f2027, #203a43, #2c5364);
  border-bottom-color: #0f2027;
  cursor: pointer;
}

button:active {
  position: relative;
  top: 1px;
}

#totalTip {
  font-size: 50px;
  margin-top: 40px;
}

#totalTip:before {
  content: "Tip Amount";
  font-size: 14px;
  font-weight: bold;
  font-family: "Poppins", sans-serif;
  display: block;
  text-transform: uppercase;
}

#totalTip sup {
  font-size: 24px;
  top: -18px;
}

#totalTip small {
  font-size: 14px;
  font-weight: bold;
  display: block;
}