body {
  background: #e0e0e0;
  font-family: Arial, sans-serif;
  text-align: center;
}

#time {
  color: #00aaff;
  font-size: 128px;
  margin: 40px auto;
}

.btn {
  display: inline-block;
  width: 100px;
  padding: 7px;
  border-radius: 5px;
  box-shadow: 0 4px 0 #0088cc;
  color: #fff;
  background: #00aaff;
  cursor: pointer;
}

.btn + .btn {
  margin-left: 10px;
}

.btn.active {
  opacity: 1.0;
}

.btn.inactive {
  opacity: 0.5;
}

body {
  background: #e0e0e0;
  font-family: Arial, sans-serif;
  text-align: center;
}

#time {
  color: #00aaff;
  font-size: 128px;
  margin: 40px auto;
}

.btn {
  display: inline-block;
  width: 100px;
  padding: 7px;
  border-radius: 5px;
  box-shadow: 0 4px 0 #0088cc;
  color: #fff;
  background: #00aaff;
  cursor: pointer;
}

.btn + .btn {
  margin-left: 10px;
}

.btn.active {
  opacity: 1.0;
}

.btn.active:hover {
  opacity: 0.7;
}

.btn.inactive {
  opacity: 0.5;
}

Enable change a color of button at hover
Enable change a color of button at hover
