styles.css

* {
  margin: 0;
  font-family: 'Anton', sans-serif;
}

body {
  background: #29294e;
  transition: background 0.5s;
}

h1 {
  width: 100%;
  text-align: center;
  color: #FFF;
  margin-top: 20px;
  font-size: 72px;
}

#container {
  width: 100%;
  display: flex;
}

#circle {
  border-radius: 50%;
  width: 340px;
  height: 340px;
  border: #FFF solid 10px;
  margin: 40px auto 0 auto;
  background: #29294e;
  font-size: 42px;
  color: #FFF;
  outline: none;
  transition: background 0.5s;
  transition: color 0.5s;
  transition: font-size 0.5s;
}

#circle:hover {
  cursor: pointer;
  background: #FFF;
  color: #29294e;
  font-size: 57px;
}

#stopwatch {
  visibility: hidden;
  font-size: 102px;
  width: 100%;
  margin: 0 auto;
  color: #29294e;
  display: none;
  text-align: center;
}

#stop {
  visibility: hidden;
  display: none;
  width: 50%;
  border: 10px #29294e solid;
  border-radius: 10px;
  font-size: 80px;
  background: #FFF;
  color: #29294e;
  margin: 130px auto 0 auto;
  transition: background 0.5s;
  transition: color 0.5s;
}

#stop:hover {
  background: #29294e;
  cursor: pointer;
  color: #FFF;
}

a {
  text-decoration: none;
  color: #FFF;
  font-size: 24px;
}

#github {
  width: 100%;
  text-align: center;
  margin-top: 70px;
}