style.css

* {
  padding: 0;
  margin: 0;
  box-sizing: border-box;
}

body {
  display: flex;
  justify-content: center;
  align-items: center;
  background: linear-gradient(to right, #21d4fd, #b721ff);
}

section {
  display: flex;
  flex-direction: column;
  justify-content: center;
  align-items: center;
}

section h1 {
  font-size: 4rem;
  font-family: sans-serif;
  margin: 20px;
  color: #fff;
  text-transform: uppercase;
}

section #timer {
  font-size: 4rem;
  margin: 20px;
}

section button {
  background: transparent;
  border: none;
  padding: 10px 20px;
}

button {
  margin: 10px;
  color: #fff;
  width: 6rem;
  cursor: pointer;
}

#start-button {
  background: #71e027;
}

#stop-button {
  background: crimson;
}

#reset-button {
  background: darkblue;
}