index.css

* {
  box-sizing: border-box;
}

body {
  background-color: #1d1f23;
  color: #ededed;
  font-family: -apple-system, BlinkMacSystemFont, 'Segoe UI', 'Roboto', 'Oxygen', 'Ubuntu', 'Cantarell', 'Fira Sans',
    'Droid Sans', 'Helvetica Neue', sans-serif;
  font-size: 24px;
  -webkit-font-smoothing: antialiased;
  -moz-osx-font-smoothing: grayscale;
}

.StopWatch {
  width: 10em;
}

.display {
  font-family: source-code-pro, Menlo, Monaco, Consolas, 'Courier New', monospace;
  font-size: 2em;
  text-align: center;
}

.controls {
  display: flex;
  justify-content: space-around;
  margin-top: 1em;
}

button {
  position: relative;
  width: 5em;
  height: 5em;
  border-radius: 100%;
  background-color: #383b4c;
  color: #fff;
  border: none;
  outline: none;
}
button:focus {
  font-weight: bold;
}
button:after {
  content: '';
  position: absolute;
  border-radius: 100%;
  top: -5px;
  left: -5px;
  right: -5px;
  bottom: -5px;
  border: #383b4c 2px solid;
}