body {
  background-image: url('https://www.osa.nchu.edu.tw/osa/safe/images/bg.jpg');
  background-repeat: repeat-x;
}

.main-area {
  margin-top: 40px;
  box-shadow: 0 0 10px rgba(0, 0, 0, .2);
  max-width: 922px;
}

.image-button {
  width: 250px;
  height: 70px;
  background-image: url('https://www.osa.nchu.edu.tw/osa/safe/images/call01.png');
  background-size: cover;
  background-position: center;
  border: none;
  cursor: pointer;
  transition: background-image 0.3s ease-in-out;
  display: block;
}

.image-button:hover {
  background-image: url('https://www.osa.nchu.edu.tw/osa/safe/images/call02.png');
}

@media (max-width: 921px) {
  .main-area {
    max-width: 100%;
  }
}

@media (max-width: 768px) {
  .main-area {
    margin-top: 20px;
    box-shadow: 0 0 5px rgba(0, 0, 0, .2);
  }

  .image-button {
    width: 125px;
    height: 35px;
    background-image: url('https://www.osa.nchu.edu.tw/osa/safe/images/call01.png');
    background-size: cover;
    background-position: center;
    border: none;
    cursor: pointer;
    transition: background-image 0.3s ease-in-out;
    display: block;
  }

  .image-button:hover {
    background-image: url('https://www.osa.nchu.edu.tw/osa/safe/images/call02.png');
  }
}

#footer-wrapper {
  background-color: #670035;
  padding: 20px 0;
}

#footer-bottom {
  color: #fff;
  font-size: 14px;
}