styles.css

* {
  font-family: sans-serif;
}

html, body, #app {
  width: 100%;
  height: 100%;
  margin: 0;
}

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

header h1 {
  font-size: 48px;
}

header h1 span {
  color: red;
}

.main {
  width: min(500px, 100%);
  display: flex;
  flex-direction: column;
  justify-content: center;
}

.main * {
  width: 100%;
}

.main .velocity-number {
  font-size: 64px;
  margin-bottom: 20px;
  text-align: center;
}

.controllers {
  display: flex;
  justify-content: flex-start;
  width: min(500px, 100%);
  margin-top: 20px;
}

.controllers .button {
  color: #333;
  cursor: pointer;
  margin-right: 20px;
  padding: 10px 10px 8px 10px;
  border: 1px solid #333;
  border-radius: 50%;
}

.controllers .button:active {
  border: 1px solid red;
}