:root {
  --ink: #1a1a1a;
  --soft-ink: #555;
  --muted: #767676;
  --accent: #4b2e83;
  --line: #e8e8e8;
  --paper: #f5f4f1;
  --font-heading: "Instrument Serif", serif;
  --grain-svg: url("data:image/svg+xml,%3Csvg viewBox='0 0 200 200' xmlns='http://www.w3.org/2000/svg'%3E%3Cfilter id='noise'%3E%3CfeTurbulence type='fractalNoise' baseFrequency='0.85' numOctaves='4' stitchTiles='stitch'/%3E%3C/filter%3E%3Crect width='100%25' height='100%25' filter='url(%23noise)' opacity='0.55'/%3E%3C/svg%3E");
}

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

html,
body {
  margin: 0;
  padding: 0;
  background: var(--paper);
  color: var(--ink);
  font-family: Inter, "PingFang TC", "Helvetica Neue", Arial, sans-serif;
  -webkit-font-smoothing: antialiased;
}

body::before {
  content: "";
  position: fixed;
  inset: 0;
  z-index: 9999;
  pointer-events: none;
  opacity: 0.7;
  mix-blend-mode: multiply;
  background-image: var(--grain-svg);
  background-size: 180px 180px;
}

.kin-main {
  position: relative;
  z-index: 0;
  width: min(42rem, 92vw);
  margin: 0 auto;
  padding: 3rem 0 3.5rem;
}

.kin-back {
  display: inline-block;
  margin-bottom: 1rem;
  font-size: 0.875rem;
  color: var(--muted);
  text-decoration: none;
  border-bottom: 1px solid transparent;
}

.kin-back:hover,
.kin-back:focus-visible {
  color: var(--accent);
  border-bottom-color: color-mix(in srgb, var(--accent) 35%, transparent);
}

.kin-title {
  margin: 0 0 0.75rem;
  font-family: var(--font-heading);
  font-size: clamp(1.55rem, 2.6vw, 2rem);
  font-weight: 400;
}

.kin-lead {
  margin: 0 0 1.25rem;
  color: var(--soft-ink);
  line-height: 1.7;
}

.kin-lead em {
  font-family: var(--font-heading);
  font-style: italic;
  color: var(--ink);
}

.kin-calc {
  border: 1px solid var(--line);
  border-radius: 14px;
  background: rgba(255, 255, 255, 0.56);
  padding: 1rem;
}

.kin-display {
  border: 1px solid var(--line);
  border-radius: 10px;
  background: #fff;
  padding: 0.85rem 0.95rem;
  min-height: 6.5rem;
  display: flex;
  flex-direction: column;
  gap: 0.45rem;
  justify-content: center;
}

.kin-chain {
  margin: 0;
  color: var(--soft-ink);
  font-size: 0.95rem;
  line-height: 1.55;
  word-break: break-all;
}

.kin-chain .kin-chain-eq {
  margin: 0 0.35rem;
  color: var(--muted);
}

.kin-result {
  margin: 0;
  font-family: var(--font-heading);
  font-size: clamp(1.7rem, 4vw, 2.2rem);
  font-weight: 400;
  line-height: 1.1;
  color: var(--ink);
  letter-spacing: 0.02em;
}

.kin-result.is-muted {
  color: var(--muted);
  font-family: inherit;
  font-size: 1rem;
  letter-spacing: 0;
}

.kin-error {
  margin: 0;
  font-size: 0.85rem;
  color: #8a2e2e;
  display: none;
}

.kin-error.is-visible {
  display: block;
}

.kin-toggles {
  display: flex;
  flex-wrap: wrap;
  gap: 0.8rem 1.25rem;
  margin: 0.9rem 0 0.8rem;
}

.kin-toggle {
  display: inline-flex;
  align-items: center;
  gap: 0.5rem;
}

.kin-toggle-label {
  font-size: 0.78rem;
  color: var(--muted);
  letter-spacing: 0.04em;
}

.kin-toggle-btn {
  appearance: none;
  border: 1px solid var(--line);
  background: #fff;
  color: var(--soft-ink);
  font-family: inherit;
  font-size: 0.9rem;
  padding: 0.32rem 0.7rem;
  cursor: pointer;
  border-radius: 999px;
  transition:
    background 140ms ease,
    color 140ms ease,
    border-color 140ms ease;
}

.kin-toggle-btn + .kin-toggle-btn {
  margin-left: -0.15rem;
}

.kin-toggle-btn:hover,
.kin-toggle-btn:focus-visible {
  border-color: color-mix(in srgb, var(--accent) 35%, var(--line));
}

.kin-toggle-btn.is-active {
  background: var(--accent);
  color: #fff;
  border-color: var(--accent);
}

.kin-keypad {
  display: grid;
  grid-template-columns: repeat(4, minmax(0, 1fr));
  gap: 0.55rem;
  margin-top: 0.25rem;
}

.kin-key {
  appearance: none;
  border: 1px solid var(--line);
  background: #fff;
  color: var(--ink);
  font-family: inherit;
  font-size: 1.05rem;
  padding: 0.7rem 0.4rem;
  cursor: pointer;
  border-radius: 10px;
  transition:
    background 140ms ease,
    color 140ms ease,
    border-color 140ms ease;
}

.kin-key:hover,
.kin-key:focus-visible {
  border-color: color-mix(in srgb, var(--accent) 35%, var(--line));
}

.kin-key:active {
  background: color-mix(in srgb, var(--accent) 8%, #fff);
}

.kin-key-util {
  color: var(--accent);
  font-weight: 600;
  font-size: 0.95rem;
}

.kin-help {
  margin-top: 1.25rem;
  color: var(--soft-ink);
  line-height: 1.7;
}

.kin-help h2 {
  margin: 1.65rem 0 0.55rem;
  color: var(--ink);
  font-size: 1rem;
}

.kin-help p {
  margin: 0 0 0.8rem;
}

.kin-help ul {
  margin: 0 0 1.25rem;
  padding-left: 1.25rem;
}

.kin-help li {
  margin: 0 0 0.45rem;
}

.kin-help em {
  font-family: var(--font-heading);
  font-style: italic;
  color: var(--ink);
}

.kin-help code {
  font-family: ui-monospace, SFMono-Regular, Menlo, Consolas, monospace;
  font-size: 0.9em;
  padding: 0.1em 0.3em;
  background: rgba(0, 0, 0, 0.04);
  border-radius: 4px;
}

.kin-help strong {
  color: var(--ink);
  font-weight: 600;
}

.kin-help-en {
  margin-top: 2.5rem;
  padding-top: 1.75rem;
  border-top: 1px solid var(--line);
}

.kin-help-en p,
.kin-help-en li {
  font-size: 0.96rem;
}

@media (max-width: 28rem) {
  .kin-key {
    font-size: 1rem;
    padding: 0.6rem 0.3rem;
  }
}