body {
  font-family: Arial, sans-serif;
  margin: 0;
  min-height: 100vh;
  display: grid;
  place-items: center;
  background: #0f172a;
  color: #e2e8f0;
}
.card {
  width: min(680px, 90vw);
  padding: 24px;
  border-radius: 16px;
  background: #111827;
  box-shadow: 0 12px 30px rgba(0,0,0,0.35);
}
.calc-grid {
  display: grid;
  grid-template-columns: 1fr 90px 1fr auto;
  gap: 12px;
  margin-top: 18px;
}
input, select, button {
  padding: 10px 12px;
  border-radius: 10px;
  border: 1px solid #334155;
  background: #1f2937;
  color: #e2e8f0;
}
button { cursor: pointer; background: #2563eb; border: none; }
.result { margin-top: 16px; }