body {
  font-family: system-ui, sans-serif;
  margin: 0;
  background: #f8f9fa;
  color: #222;
}
header {
  background: #234;
  color: #fff;
  padding: 1rem;
  margin-bottom: 2rem;
}
nav button {
  margin: 0 0.5rem;
  padding: 0.5rem 1rem;
  background: #fff;
  border: none;
  border-radius: 0.3rem;
  cursor: pointer;
}
nav button:hover {
  background: #f0f0f0;
}
.topic {
  background: #fff;
  margin: 1rem auto;
  max-width: 600px;
  border-radius: 0.5rem;
  box-shadow: 0 1px 6px #0001;
  padding: 1rem;
}
.topic h2 {
  margin: 0 0 0.5rem;
  color: #235;
}
.topic .code {
  font-size: 0.95em;
  color: #568;
}