:root{
  --paper: #eceee7;
  --paper-raised: #f6f7f2;
  --ink: #1e2530;
  --ink-soft: #4a5262;
  --line-hair: #cbcfc4;
  --accent: #2b4c7e;

  --c-ones: #2166ac;
  --c-tens: #3f9142;
  --c-hundreds: #c98a2b;
  --c-thousands: #7b4b94;
  --c-tenthousands: #c9506e;
  --c-overflow: #5b6470;

  --font-display: "Space Grotesk", "Segoe UI", sans-serif;
  --font-body: "Source Sans 3", "Segoe UI", sans-serif;
  --font-mono: "IBM Plex Mono", "Courier New", monospace;

  --radius: 10px;
}

*{box-sizing:border-box;}
html{scroll-behavior:smooth;}
body{
  margin:0;
  background:
    repeating-linear-gradient(0deg, transparent 0, transparent 27px, rgba(30,37,48,0.035) 27px, rgba(30,37,48,0.035) 28px),
    repeating-linear-gradient(90deg, transparent 0, transparent 27px, rgba(30,37,48,0.035) 27px, rgba(30,37,48,0.035) 28px),
    var(--paper);
  color:var(--ink);
  font-family:var(--font-body);
  line-height:1.55;
}

.wrap{max-width:920px; margin:0 auto; padding:0 20px;}

.visually-hidden{
  position:absolute; width:1px; height:1px; overflow:hidden;
  clip:rect(0,0,0,0); white-space:nowrap;
}

/* ---------- header ---------- */
.site-header{padding:44px 0 28px;}
.eyebrow{
  font-family:var(--font-mono);
  text-transform:uppercase;
  letter-spacing:0.14em;
  font-size:0.78rem;
  color:var(--accent);
  margin:0 0 10px;
}
.site-header h1{
  font-family:var(--font-display);
  font-size:clamp(1.9rem, 4vw, 2.9rem);
  line-height:1.1;
  margin:0 0 14px;
  max-width:16ch;
}
.subhead{
  max-width:56ch;
  color:var(--ink-soft);
  font-size:1.05rem;
  margin:0;
}

/* ---------- ad slots ---------- */
.ad-slot{
  margin:28px auto;
  min-height:90px;
  border:1px dashed var(--line-hair);
  border-radius:var(--radius);
  display:flex;
  align-items:center;
  justify-content:center;
  background:rgba(30,37,48,0.02);
}
.ad-slot__label{
  font-family:var(--font-mono);
  font-size:0.7rem;
  letter-spacing:0.08em;
  text-transform:uppercase;
  color:var(--ink-soft);
  opacity:0.6;
}
/* Fixed heights reserve space so loading ads don't shift the page (protects Core Web Vitals / CLS). */
.ad-slot--leaderboard{max-width:970px; height:90px;}
.ad-slot--inline{max-width:728px; height:90px;}
.ad-slot--bottom{max-width:970px; height:90px;}

@media (max-width: 640px){
  .ad-slot--leaderboard,
  .ad-slot--inline,
  .ad-slot--bottom{height:100px;}
}

/* ---------- app panel ---------- */
.app-section{margin:8px auto 40px;}
.panel{
  background:var(--paper-raised);
  border:1px solid var(--line-hair);
  border-radius:16px;
  padding:clamp(18px, 3vw, 36px);
  box-shadow:0 1px 0 rgba(30,37,48,0.04);
}

.numbers-row{
  display:flex;
  align-items:center;
  justify-content:center;
  gap:20px;
  flex-wrap:wrap;
  margin-bottom:18px;
}
.number-panel{text-align:center;}
.panel-label{
  font-family:var(--font-mono);
  font-size:0.82rem;
  text-transform:uppercase;
  letter-spacing:0.06em;
  color:var(--ink-soft);
  margin:0 0 10px;
}
.panel-sub{color:var(--ink-soft); opacity:0.7; text-transform:none; letter-spacing:0;}

.operator{
  font-family:var(--font-display);
  font-size:2rem;
  color:var(--accent);
  padding-top:22px;
}

.digit-row{display:flex; gap:8px;}

.digit-box{
  display:flex;
  flex-direction:column;
  align-items:center;
  width:64px;
}
.digit-box .place-tag{
  font-family:var(--font-mono);
  font-size:0.62rem;
  text-transform:uppercase;
  letter-spacing:0.05em;
  color:var(--ink-soft);
  margin-top:6px;
}
.digit-box.place-hundreds .digit-input{border-color:var(--c-hundreds); color:var(--c-hundreds);}
.digit-box.place-tens .digit-input{border-color:var(--c-tens); color:var(--c-tens);}
.digit-box.place-ones .digit-input{border-color:var(--c-ones); color:var(--c-ones);}
.digit-box.place-hundreds .spin{color:var(--c-hundreds);}
.digit-box.place-tens .spin{color:var(--c-tens);}
.digit-box.place-ones .spin{color:var(--c-ones);}

.spin{
  width:36px; height:26px;
  border:1px solid var(--line-hair);
  background:var(--paper-raised);
  border-radius:6px;
  font-family:var(--font-mono);
  font-size:0.85rem;
  cursor:pointer;
  line-height:1;
}
.spin:hover{background:var(--paper);}
.spin:active{transform:translateY(1px);}
.spin.up{border-bottom-left-radius:0; border-bottom-right-radius:0; border-bottom:none;}
.spin.down{border-top-left-radius:0; border-top-right-radius:0;}

.digit-input{
  width:36px; height:44px;
  text-align:center;
  font-family:var(--font-mono);
  font-weight:600;
  font-size:1.3rem;
  border:2px solid var(--line-hair);
  border-radius:0;
  background:#fff;
  -moz-appearance:textfield;
}
.digit-input::-webkit-outer-spin-button,
.digit-input::-webkit-inner-spin-button{-webkit-appearance:none; margin:0;}
.digit-input:focus{outline:2px solid var(--accent); outline-offset:2px;}

.presets{
  display:flex;
  flex-wrap:wrap;
  gap:8px;
  align-items:center;
  justify-content:center;
  margin:6px 0 26px;
}
.presets__label{
  font-family:var(--font-mono);
  font-size:0.78rem;
  color:var(--ink-soft);
}
.preset-btn{
  font-family:var(--font-mono);
  font-size:0.8rem;
  padding:6px 12px;
  border:1px solid var(--line-hair);
  background:#fff;
  border-radius:20px;
  cursor:pointer;
  color:var(--ink);
}
.preset-btn:hover{border-color:var(--accent); color:var(--accent);}

/* ---------- step-through walkthrough ---------- */
.stepper{
  display:flex;
  justify-content:center;
  margin:0 0 14px;
}
.step-nav{
  display:flex;
  align-items:center;
  gap:10px;
  flex-wrap:wrap;
  justify-content:center;
}
.step-btn{
  font-family:var(--font-mono);
  font-size:0.82rem;
  padding:8px 16px;
  border:1px solid var(--accent);
  background:var(--accent);
  color:#fff;
  border-radius:20px;
  cursor:pointer;
}
.step-btn:hover{filter:brightness(1.1);}
.step-btn:disabled{opacity:0.35; cursor:default; filter:none;}
.step-btn--nav, .step-btn--exit{
  background:#fff;
  color:var(--accent);
}
.step-btn--exit{border-color:var(--line-hair); color:var(--ink-soft);}
.step-counter{
  font-family:var(--font-mono);
  font-size:0.78rem;
  color:var(--ink-soft);
  min-width:8.5em;
  text-align:center;
}
.step-caption{
  max-width:60ch;
  margin:0 auto 16px;
  padding:12px 16px;
  background:rgba(43,76,126,0.07);
  border-left:3px solid var(--accent);
  border-radius:6px;
  font-size:0.95rem;
  color:var(--ink);
}

.noscript-fallback{
  padding:18px;
  border:1px solid var(--line-hair);
  border-radius:var(--radius);
  background:#fff;
  font-size:0.95rem;
  color:var(--ink-soft);
}

tr.row-focus td{background:rgba(43,76,126,0.09);}

.diagram-wrap{
  max-width:560px;
  margin:0 auto 18px;
  aspect-ratio:1/1;
}
#diagram{width:100%; height:100%; display:block;}

.legend{
  display:flex;
  gap:32px;
  flex-wrap:wrap;
  justify-content:center;
  margin:8px 0 26px;
}
.legend-title{
  font-family:var(--font-mono);
  font-size:0.72rem;
  text-transform:uppercase;
  letter-spacing:0.04em;
  color:var(--ink-soft);
  margin:0 0 6px;
  text-align:center;
}
.legend-list{
  display:flex;
  gap:14px;
  list-style:none;
  margin:0;
  padding:0;
  flex-wrap:wrap;
  justify-content:center;
  font-size:0.85rem;
}
.legend-list li{display:flex; align-items:center; gap:6px;}
.swatch{width:12px; height:12px; border-radius:3px; display:inline-block;}

.result{
  border-top:1px solid var(--line-hair);
  padding-top:22px;
}
.equation{
  font-family:var(--font-mono);
  font-size:clamp(1.3rem, 4vw, 1.9rem);
  text-align:center;
  margin:0 0 22px;
}
.equation span{font-weight:700;}

.table-scroll{overflow-x:auto;}
table#breakdown{
  width:100%;
  border-collapse:collapse;
  font-family:var(--font-mono);
  font-size:0.85rem;
  min-width:560px;
}
table#breakdown caption{
  text-align:left;
  font-family:var(--font-body);
  color:var(--ink-soft);
  margin-bottom:8px;
  font-size:0.9rem;
}
table#breakdown th, table#breakdown td{
  border-bottom:1px solid var(--line-hair);
  padding:8px 10px;
  text-align:left;
}
table#breakdown th{color:var(--ink-soft); font-weight:600;}
table#breakdown td.place-cell{font-weight:700;}

/* ---------- content sections ---------- */
.content-section{margin:44px auto;}
.content-section h2{
  font-family:var(--font-display);
  font-size:1.5rem;
  margin:0 0 14px;
}
.content-section h3{
  font-family:var(--font-display);
  font-size:1.1rem;
  margin:0 0 6px;
}
.content-section p{max-width:70ch; color:var(--ink-soft);}
.explainer-list{max-width:70ch; padding-left:1.2em; color:var(--ink-soft);}
.explainer-list li{margin-bottom:10px;}
.explainer-list strong{color:var(--ink);}

.faq{display:grid; gap:20px;}
.faq-item p{margin:0;}

.site-footer{
  border-top:1px solid var(--line-hair);
  padding:26px 0 50px;
  color:var(--ink-soft);
  font-size:0.85rem;
}

/* ---------- responsive ---------- */
@media (max-width: 640px){
  .numbers-row{gap:14px;}
  .operator{padding-top:0;}
  .digit-box{width:52px;}
  .digit-input{width:32px; height:38px; font-size:1.1rem;}
  .spin{width:32px;}
  .legend{gap:20px;}
}

@media (prefers-reduced-motion: reduce){
  html{scroll-behavior:auto;}
}