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

body {
  font-family: system-ui, -apple-system, sans-serif;
  background: #f5f5f5;
  color: #111;
  min-height: 100vh;
  display: flex;
  align-items: center;
  justify-content: center;
  padding: 2rem;
}

main {
  text-align: center;
  max-width: 480px;
  width: 100%;
}

h1 {
  font-size: 2.5rem;
  font-weight: 700;
  margin-bottom: 0.5rem;
}

.subtitle {
  color: #555;
  margin-bottom: 2rem;
  font-size: 1.05rem;
}

.card {
  background: #fff;
  border-radius: 12px;
  padding: 2rem;
  box-shadow: 0 2px 12px rgba(0, 0, 0, 0.08);
  display: flex;
  flex-direction: column;
  align-items: center;
  gap: 1.5rem;
}

label {
  font-size: 1rem;
  font-weight: 500;
  color: #333;
}

select {
  appearance: none;
  background-color: #f0f0f0;
  border: 1px solid #ccc;
  border-radius: 8px;
  padding: 10px 40px 10px 16px;
  font-size: 1.1rem;
  color: #333;
  cursor: pointer;
  outline: none;
  transition: border-color 0.2s, box-shadow 0.2s;
  width: 180px;
  background-image: url('data:image/svg+xml;charset=UTF-8,<svg xmlns="http://www.w3.org/2000/svg" viewBox="0 0 24 24" fill="none" stroke="%23333" stroke-width="2" stroke-linecap="round" stroke-linejoin="round"><polyline points="6 9 12 15 18 9"></polyline></svg>');
  background-repeat: no-repeat;
  background-position: right 12px center;
  background-size: 16px;
}

select:hover {
  background-color: #e8e8e8;
}

select:focus {
  border-color: #3b82f6;
  box-shadow: 0 0 0 3px rgba(59, 130, 246, 0.2);
}

.result {
  display: flex;
  align-items: baseline;
  gap: 0.4rem;
}

.result.hidden {
  visibility: hidden;
}

.ppm {
  font-size: 3.5rem;
  font-weight: 700;
  color: #2563eb;
  line-height: 1;
}

.unit {
  font-size: 1.2rem;
  color: #555;
  font-weight: 500;
}

footer {
  margin-top: 2rem;
}

footer p {
  font-size: 0.75rem;
  color: #999;
}