:root{
  --bg:#0f1117;--sf:#1a1d27;--sf2:#222635;--bd:#2e3347;
  --tx:#e2e4ed;--txm:#8b8fa7;--tx2:#b6b9cc;
  --xl:#22c55e;          /* Excel green */
  --xl-soft:rgba(34,197,94,.15);
  --xl-line:rgba(34,197,94,.35);
  --mod1:#22d3ee; --mod2:#a78bfa;
  --grn:#22c55e; --red:#ef4444; --yel:#eab308;
  --cbg:#141620;
  --code-k:#c084fc; --code-t:#22d3ee; --code-s:#34d399;
  --code-c:#5c6078; --code-n:#fb923c;
  /* Excel-grid (light) */
  --gbg:#fdfdfd; --gtx:#1a1d27; --gbd:#d0d7de; --ghd:#f1f3f5; --ghdtx:#5a6068;
  --gsel-bg:#d4edda; --gsel-bd:#217346;
}
*{margin:0;padding:0;box-sizing:border-box}
html{scroll-behavior:smooth;scroll-padding-top:80px}
body{
  background:var(--bg);color:var(--tx);
  font-family:'DM Sans',sans-serif;line-height:1.7;
}

/* ======================= Topbar ======================= */
.topbar{
  position:sticky;top:0;z-index:50;
  display:flex;align-items:center;justify-content:space-between;
  padding:.8rem 1.5rem;
  background:rgba(15,17,23,.85);backdrop-filter:blur(12px);
  border-bottom:1px solid var(--bd);
}
.brand{display:flex;align-items:center;gap:.7rem}
.logo{
  display:inline-flex;align-items:center;justify-content:center;
  width:30px;height:30px;border-radius:7px;
  background:var(--xl);color:#0d2818;
  font-family:'JetBrains Mono',monospace;font-weight:700;font-size:1rem;
}
.brand-name{font-family:'JetBrains Mono',monospace;font-weight:700;font-size:1.05rem;letter-spacing:-.3px}
.brand-name .accent{color:var(--xl)}
.progress{display:flex;align-items:center;gap:.8rem}
.progress-bar{
  width:140px;height:6px;background:var(--sf2);border-radius:99px;overflow:hidden;
}
.progress-fill{
  height:100%;width:0%;background:linear-gradient(90deg,var(--xl),#4ade80);
  transition:width .35s ease;border-radius:99px;
}
.progress-text{font-family:'JetBrains Mono',monospace;font-size:.78rem;color:var(--txm)}
.progress-text strong{color:var(--xl)}
#reset-btn{
  background:transparent;border:1px solid var(--bd);color:var(--txm);
  width:30px;height:30px;border-radius:7px;cursor:pointer;font-size:1rem;
  transition:all .2s;
}
#reset-btn:hover{border-color:var(--red);color:var(--red)}

/* ======================= Layout ======================= */
.layout{display:flex;align-items:flex-start;max-width:1400px;margin:0 auto}

/* ======================= Sidebar ======================= */
.sidebar{
  position:sticky;top:62px;
  width:280px;flex-shrink:0;
  height:calc(100vh - 62px);overflow-y:auto;
  padding:1.5rem 1rem;border-right:1px solid var(--bd);
}
.side-section{margin-bottom:1.5rem}
.side-section h3{
  font-family:'JetBrains Mono',monospace;font-size:.72rem;
  text-transform:uppercase;letter-spacing:1.5px;
  color:var(--txm);margin-bottom:.6rem;padding:0 .5rem;
}
.toc{list-style:none;padding:0}
.toc li{margin:.1rem 0}
.toc a{
  display:flex;align-items:center;gap:.6rem;
  padding:.45rem .6rem;border-radius:6px;
  color:var(--tx2);font-size:.85rem;text-decoration:none;
  transition:all .15s;
  border-left:2px solid transparent;
}
.toc a:hover{background:var(--sf);color:var(--tx)}
.toc a.active{
  background:var(--xl-soft);color:var(--tx);
  border-left-color:var(--xl);
}
.toc a.done .ln::after{content:" ✓";color:var(--xl)}
.toc a.done{color:var(--txm)}
.ln{
  font-family:'JetBrains Mono',monospace;font-size:.7rem;
  color:var(--txm);min-width:30px;
}
.toc a.active .ln,.toc a:hover .ln{color:var(--xl)}

/* ======================= Content ======================= */
.content{flex:1;min-width:0;padding:2rem 2.5rem 5rem;max-width:900px}

/* Hero */
.hero{
  text-align:center;padding:2rem 0 3rem;border-bottom:1px solid var(--bd);
  margin-bottom:2.5rem;position:relative;
}
.hero::before{
  content:'';position:absolute;top:-30%;left:50%;transform:translateX(-50%);
  width:500px;height:500px;
  background:radial-gradient(circle,rgba(34,197,94,.07),transparent 70%);
  pointer-events:none;z-index:-1;
}
.hero h1{
  font-family:'JetBrains Mono',monospace;
  font-size:clamp(1.6rem,3.5vw,2.2rem);font-weight:700;
  letter-spacing:-.5px;margin-bottom:.7rem;
}
.hero h1 span{color:var(--xl)}
.hero p{color:var(--txm);font-size:.95rem;max-width:560px;margin:0 auto}

/* ======================= Lessons ======================= */
.lesson{
  background:var(--sf);border:1px solid var(--bd);
  border-radius:14px;padding:1.6rem 1.8rem 1.4rem;
  margin-bottom:1.8rem;
  scroll-margin-top:80px;
  transition:border-color .25s;
}
.lesson.done{border-color:var(--xl-line)}
.lesson-head{
  display:flex;align-items:center;gap:.8rem;flex-wrap:wrap;
  margin-bottom:.6rem;
}
.lesson-num{
  font-family:'JetBrains Mono',monospace;font-weight:700;font-size:.75rem;
  padding:.3rem .6rem;border-radius:6px;
  background:var(--xl-soft);color:var(--xl);
  white-space:nowrap;
}
.lesson-head h2{
  flex:1;min-width:200px;
  font-size:1.25rem;font-weight:700;letter-spacing:-.3px;
}
.badge{
  font-family:'JetBrains Mono',monospace;font-size:.65rem;
  padding:.25rem .55rem;border-radius:5px;
  text-transform:uppercase;letter-spacing:.5px;
}
.badge.mod-1{background:rgba(34,211,238,.12);color:var(--mod1)}
.badge.mod-2{background:rgba(167,139,250,.12);color:var(--mod2)}
.lead{color:var(--tx2);font-size:.95rem;margin-bottom:1rem}
.lesson h4{
  font-family:'JetBrains Mono',monospace;font-size:.72rem;
  text-transform:uppercase;letter-spacing:1.2px;color:var(--txm);
  margin:1.4rem 0 .55rem;
}
.lesson p,.lesson li{font-size:.92rem}
.lesson ul{padding-left:1.3rem;margin-bottom:.4rem}
.lesson ul li{margin:.25rem 0}
.lesson ul li::marker{color:var(--xl)}
.lesson b{color:var(--tx);font-weight:600}
.note{color:var(--txm);font-size:.85rem;margin-top:.5rem;font-style:italic}

/* Inline code */
code{
  font-family:'JetBrains Mono',monospace;font-size:.85em;
  background:var(--cbg);padding:.1rem .35rem;border-radius:4px;
  color:#c9cce0;border:1px solid var(--bd);
}

/* Pre / code blocks */
pre{
  background:var(--cbg);border:1px solid var(--bd);border-radius:8px;
  padding:1rem 1.2rem;overflow-x:auto;
  font-family:'JetBrains Mono',monospace;font-size:.82rem;line-height:1.7;
  margin:.7rem 0;color:#c9cce0;
}
.k{color:var(--code-k)}.t{color:var(--code-t)}.s{color:var(--code-s)}
.c{color:var(--code-c);font-style:italic}.n{color:var(--code-n)}

/* Keyboard */
kbd{
  font-family:'JetBrains Mono',monospace;font-size:.78em;
  background:var(--sf2);border:1px solid var(--bd);
  border-bottom-width:2px;border-radius:5px;
  padding:.1rem .4rem;color:var(--tx);
  display:inline-block;line-height:1.4;
}

/* Callout */
.callout{
  margin-top:.5rem;padding:.85rem 1rem;
  background:var(--xl-soft);border-left:3px solid var(--xl);
  border-radius:0 8px 8px 0;
  font-size:.9rem;color:var(--tx);
}
.callout code{background:rgba(0,0,0,.25)}

/* ======================= Excel grid (light themed) ======================= */
.xl{
  border-collapse:separate;border-spacing:0;
  background:var(--gbg);color:var(--gtx);
  font-family:'DM Sans',sans-serif;font-size:.85rem;
  margin:.8rem 0;border:1px solid #b3b3b3;border-radius:4px;
  overflow:hidden;
  box-shadow:0 1px 0 rgba(255,255,255,.04);
}
.xl tr:first-child th:first-child{border-top-left-radius:4px}
.xl th,.xl td{
  border-right:1px solid var(--gbd);border-bottom:1px solid var(--gbd);
  padding:.32rem .65rem;min-width:75px;
  text-align:left;line-height:1.4;
}
.xl tr:last-child th,.xl tr:last-child td{border-bottom:none}
.xl th:last-child,.xl td:last-child{border-right:none}
.xl thead th,.xl tr th:first-child,.xl tr th{
  background:var(--ghd);color:var(--ghdtx);
  font-weight:600;font-size:.75rem;text-align:center;
  min-width:32px;
}
.xl tbody th{min-width:32px}
.xl td.sel{
  background:var(--gsel-bg);
  outline:2px solid var(--gsel-bd);outline-offset:-2px;
  font-weight:600;
}
.xl td{font-family:'DM Sans',sans-serif}
.xl td:has(>code), .xl code{font-family:'JetBrains Mono',monospace;font-size:.82em}

/* Formula bar */
.fxbar{
  display:flex;align-items:center;
  background:var(--gbg);color:var(--gtx);
  border:1px solid #b3b3b3;border-radius:4px;
  padding:.45rem .8rem;margin:.6rem 0;
  font-family:'JetBrains Mono',monospace;font-size:.85rem;
}
.fxbar::before{
  content:'fx';font-style:italic;color:#8a8a8a;
  margin-right:.7rem;padding-right:.7rem;
  border-right:1px solid var(--gbd);
}

/* ======================= Exercises ======================= */
.exercises-block{margin-top:1.4rem}
.exercises-block h4{
  display:flex;align-items:center;gap:.5rem;
  margin-bottom:.7rem;
}
.ex-count{
  font-family:'JetBrains Mono',monospace;font-size:.7rem;
  background:var(--xl-soft);color:var(--xl);
  padding:.15rem .45rem;border-radius:5px;
  text-transform:none;letter-spacing:0;
}
.ex-list{display:flex;flex-direction:column;gap:.7rem}
.ex{
  background:var(--sf2);border:1px solid var(--bd);
  border-radius:9px;padding:.85rem 1rem;
  transition:border-color .2s;
}
.ex:hover{border-color:var(--xl-line)}
.ex-row{
  display:flex;align-items:flex-start;gap:.6rem;
  margin-bottom:.5rem;
}
.ex-n{
  font-family:'JetBrains Mono',monospace;font-weight:700;
  font-size:.72rem;
  background:var(--cbg);color:var(--xl);
  padding:.2rem .5rem;border-radius:5px;
  flex-shrink:0;line-height:1.4;
  border:1px solid var(--bd);
  margin-top:.05rem;
}
.ex-q{flex:1;font-size:.92rem;color:var(--tx);line-height:1.65}
.ex-q code{background:rgba(0,0,0,.3);font-size:.85em}
.diff{
  font-size:.65rem;font-family:'JetBrains Mono',monospace;
  padding:.2rem .5rem;border-radius:4px;
  text-transform:uppercase;letter-spacing:.5px;
  flex-shrink:0;line-height:1.4;
  margin-top:.05rem;
}
.d-e{background:rgba(34,197,94,.15);color:var(--grn)}
.d-m{background:rgba(234,179,8,.15);color:var(--yel)}
.d-h{background:rgba(239,68,68,.15);color:var(--red)}
.ex-sol-btn{
  display:inline-flex;align-items:center;gap:.4rem;
  padding:.35rem .8rem;border-radius:6px;
  background:rgba(34,197,94,.08);
  border:1px solid rgba(34,197,94,.25);
  color:var(--xl);
  font-family:'JetBrains Mono',monospace;font-size:.72rem;
  cursor:pointer;transition:all .2s;
  margin-left:calc(.72rem + .65rem + 1.1rem);  /* align under question */
}
.ex-sol-btn::before{content:'✓';font-size:.8rem}
.ex-sol-btn:hover{background:rgba(34,197,94,.15)}
.ex-sol-btn.open{
  background:transparent;color:var(--txm);border-color:var(--bd);
}
.ex-sol-btn.open::before{content:'×'}
.ex-sol{
  display:none;margin-top:.6rem;
  padding:.8rem 1rem;
  background:rgba(34,197,94,.06);
  border-left:3px solid var(--xl);border-radius:0 8px 8px 0;
  font-size:.88rem;color:var(--tx2);line-height:1.7;
  margin-left:calc(.72rem + .65rem + 1.1rem);
}
.ex-sol.show{display:block;animation:exFade .25s ease}
.ex-sol code{background:rgba(0,0,0,.35);color:#c9cce0}
.ex-sol .note{display:block;margin-top:.4rem;font-size:.82rem}
@keyframes exFade{
  from{opacity:0;transform:translateY(-4px)}
  to{opacity:1;transform:none}
}
.ex-load-error{
  margin-top:1rem;padding:.7rem 1rem;
  background:rgba(239,68,68,.08);border-left:3px solid var(--red);
  border-radius:0 8px 8px 0;
  font-size:.85rem;color:var(--txm);
}

/* ======================= Done button ======================= */
.done-btn{
  margin-top:1.4rem;
  display:inline-flex;align-items:center;gap:.5rem;
  padding:.55rem 1.1rem;
  background:transparent;border:1px solid var(--bd);
  border-radius:7px;color:var(--txm);
  font-family:'JetBrains Mono',monospace;font-size:.78rem;
  cursor:pointer;transition:all .2s;
}
.done-btn:hover{border-color:var(--xl);color:var(--xl)}
.done-btn::before{content:'○';font-size:1rem;line-height:1}
.lesson.done .done-btn{
  background:var(--xl-soft);border-color:var(--xl);color:var(--xl);
}
.lesson.done .done-btn::before{content:'✓'}

/* ======================= End card ======================= */
.end-card{
  text-align:center;padding:3rem 1.5rem;
  background:linear-gradient(180deg,var(--xl-soft),transparent);
  border:1px solid var(--xl-line);border-radius:14px;
  margin-top:2rem;
}
.end-card h2{
  font-family:'JetBrains Mono',monospace;font-size:1.4rem;
  margin-bottom:.6rem;color:var(--xl);
}
.end-card p{color:var(--tx2);font-size:.95rem;max-width:520px;margin:0 auto}

/* ======================= Responsive ======================= */
@media (max-width:1000px){
  .sidebar{
    position:static;width:100%;height:auto;
    border-right:none;border-bottom:1px solid var(--bd);
    padding:1rem;
  }
  .layout{flex-direction:column}
  .content{padding:1.5rem 1.2rem 4rem;max-width:100%}
  .toc{display:grid;grid-template-columns:repeat(auto-fill,minmax(190px,1fr));gap:.2rem}
}
@media (max-width:600px){
  .topbar{padding:.6rem .9rem}
  .brand-name{font-size:.95rem}
  .progress-bar{width:80px}
  .lesson{padding:1.2rem 1.1rem 1rem}
  .lesson-head h2{font-size:1.1rem}
  .xl{font-size:.78rem}
  .xl th,.xl td{padding:.28rem .45rem;min-width:55px}
  pre{font-size:.74rem;padding:.7rem .8rem}
}