body {
  background-color: #eee;
  font-family: Calibri, Arial, sans-serif;
}
#counter {
  width: 50%;
  background: #ddd;
  border: 2px solid #009afd;
  border-radius: 5px;
  padding: 5px;
  text-align: center;
  transition: .3s;
  margin: 0 auto;
}
#counter.pauseInterval {
  border-color: red;  
}