#main_container {
  width: 700px;
  margin: 20px;
  margin-left: auto;
  margin-right: auto;
  padding: 30px;
  font-family: roman;
  border-radius: 20px;
  border: 3px solid #999;
}

#birth_date_input,
#age_container {
  text-align: center;
  margin: 20px;
  margin-left: auto;
  margin-right: auto;
}

#age_container {
  margin: 40px 5px;
  padding: 20px;
  border-radius: 50px;
  border: 1px solid #000;
  font-weight: bolder;
  background: #eee;
  font-size: 20px;
  line-height: 40px;
}

#calculate {
  cursor: pointer;
  text-align: center;
  width: 300px;
  padding: 5px;
  margin: 50px;
  margin-left: auto;
  margin-right: auto;
  border: 5px solid #999;
  border-radius: 100px;
  background: #de3163;
  background: -webkit-gradient(
    linear,
    left top,
    left bottom,
    from(#de3163),
    to(#de3163)
  );
  background: -moz-linear-gradient(top, #de3163, #de3163);
  font-weight: bold;
  height: 38px;
  box-shadow: 0 -8px inset;
}

#calculate:hover {
  background: -webkit-gradient(
    linear,
    left top,
    left bottom,
    from(#de3163),
    to(#de3163)
  );
  background: -moz-linear-gradient(top, #de3163, #de3163);
  margin-top: 50px;
  height: 25px;
  box-shadow: 0 -5px inset;
}

#calculate:active {
  background: #ff00ff;
  padding-top: 20px;
  height: 15px;
  box-shadow: 0 10px #000 inset;
}

#age {
  padding: 15px;
  border: 5px dashed rgb(252, 4, 4);
  background-color: #28b463;
}

html {
  background: url("https://images.hdqwalls.com/download/lake-minimal-green-4k-c0-3840x2160.jpg")
    no-repeat center center fixed;
  -webkit-background-size: cover;
  -moz-background-size: cover;
  -o-background-size: cover;
  background-size: cover;
}