:root {
  --bg: #f6f6f3; --surface: #fcfcfb; --ink: #0b0b0b; --muted: #6b6963;
  --line: #e1e0d9; --line-strong: #c3c2b7; --on-ink: #fcfcfb; --danger: #d03b3b;
  --font-body: "IBM Plex Sans", system-ui, -apple-system, "Segoe UI", sans-serif;
  --font-num: "IBM Plex Mono", ui-monospace, SFMono-Regular, Menlo, Consolas, monospace;
  color-scheme: light;
}
@media (prefers-color-scheme: dark) {
  :root:not([data-theme="light"]) {
    --bg: #0d0d0d; --surface: #1a1a19; --ink: #f4f4f1; --muted: #9a988f;
    --line: #2c2c2a; --line-strong: #43433f; --on-ink: #0d0d0d; --danger: #e66767;
    color-scheme: dark;
  }
}
*, *::before, *::after { box-sizing: border-box; }
html, body { height: 100%; }
body {
  margin: 0; padding: 16px; display: grid; place-items: center;
  background: var(--bg); color: var(--ink); font: 15px/1.5 var(--font-body);
}
.card {
  width: min(22rem, 100%); background: var(--surface); border: 1px solid var(--line);
  border-radius: 12px; padding: 28px 24px; text-align: center;
}
h1 { margin: 0; font-size: 1.25rem; font-weight: 600; letter-spacing: -0.01em; }
.muted { margin: 4px 0 20px; color: var(--muted); }
.sr { position: absolute; width: 1px; height: 1px; overflow: hidden; clip: rect(0 0 0 0); white-space: nowrap; }
form { display: flex; flex-direction: column; gap: 12px; }
input {
  width: 100%; min-height: 56px; padding: 8px 12px; text-align: center;
  font: 500 28px var(--font-num); letter-spacing: 0.6em; text-indent: 0.6em;
  color: var(--ink); background: var(--bg); border: 1px solid var(--line-strong); border-radius: 10px;
}
input:focus-visible, .btn:focus-visible { outline: 2px solid var(--ink); outline-offset: 2px; }
.btn {
  min-height: 44px; border-radius: 8px; border: 1px solid var(--ink);
  background: var(--ink); color: var(--on-ink); font: 600 15px var(--font-body); cursor: pointer;
}
.btn:disabled { opacity: 0.5; cursor: not-allowed; }
.msg { min-height: 1.5em; margin: 12px 0 0; font-size: 0.875rem; color: var(--muted); }
.msg.err { color: var(--danger); }