:root {
  --bg:#0e1117;--surface:#13161f;--card:#181c28;--border:#1f2535;--border2:#2a3248;
  --text:#e8ecf5;--body:#8b95b0;--muted:#4a5470;--dim:#1a2038;
  --p1:#34d399;--p2:#059669;
  --grad:linear-gradient(135deg,#10b981,#3b82f6);
  --green:#4ec98a;--green-dim:rgba(78,201,138,0.10);--green-border:rgba(78,201,138,0.22);
  --yellow:#e8c84e;--yellow-dim:rgba(232,200,78,0.10);--yellow-border:rgba(232,200,78,0.22);
  --red:#e8634e;--red-dim:rgba(232,99,78,0.10);--red-border:rgba(232,99,78,0.22);
  --blue:#4e8ce8;--blue-dim:rgba(78,140,232,0.10);--blue-border:rgba(78,140,232,0.22);
  --orange:#e8974e;
  --accent1:#a78bfa;--accent2:#34d399;--accent3:#f59e0b;--accent4:#60a5fa;
  --accent5:#f87171;--accent6:#c084fc;--accent7:#fb923c;--accent8:#38bdf8;
}
*,*::before,*::after{box-sizing:border-box;margin:0;padding:0}
html{scroll-behavior:smooth}
body{font-family:'Inter',sans-serif;background:var(--bg);color:var(--text);line-height:1.6;overflow-x:hidden}

header{background:var(--surface);border-bottom:1px solid var(--border);padding:0 40px;height:58px;display:flex;align-items:center;justify-content:space-between;position:sticky;top:0;z-index:200;backdrop-filter:blur(12px)}
.logo{display:flex;align-items:center;gap:10px}
.logo-icon{width:30px;height:30px;border-radius:7px;background:var(--grad);display:flex;align-items:center;justify-content:center;font-size:15px}
.logo-text{font-size:14px;font-weight:700;letter-spacing:-.01em}
.logo-text span{background:var(--grad);-webkit-background-clip:text;-webkit-text-fill-color:transparent;background-clip:text}
.hdr-badge{font-size:10px;font-weight:700;letter-spacing:.08em;text-transform:uppercase;padding:3px 9px;background:rgba(52,211,153,.1);border:1px solid rgba(52,211,153,.22);border-radius:4px;color:var(--accent2)}
.prog-wrap{display:flex;align-items:center;gap:10px}
.prog-label{font-family:'JetBrains Mono',monospace;font-size:11px;color:var(--muted)}
.prog-track{width:110px;height:4px;background:var(--border2);border-radius:2px;overflow:hidden}
.prog-fill{height:100%;background:var(--grad);border-radius:2px;transition:width .5s ease;width:14.3%}
.prog-pct{font-family:'JetBrains Mono',monospace;font-size:11px;font-weight:700;background:var(--grad);-webkit-background-clip:text;-webkit-text-fill-color:transparent;background-clip:text;min-width:30px}

.shell{display:flex;min-height:calc(100vh - 58px)}
.sidebar{width:240px;flex-shrink:0;border-right:1px solid var(--border);background:var(--surface);position:sticky;top:58px;height:calc(100vh - 58px);overflow-y:auto;display:flex;flex-direction:column}
.sb-hdr{padding:14px 16px 10px;border-bottom:1px solid var(--border);display:flex;align-items:center;justify-content:space-between}
.sb-title{font-size:11px;font-weight:700;letter-spacing:.08em;text-transform:uppercase;color:var(--muted)}
.sb-count{font-family:'JetBrains Mono',monospace;font-size:10px;color:var(--muted);background:var(--dim);padding:2px 7px;border-radius:4px}
.nav-list{padding:6px 0;flex:1}
.nav-btn{display:flex;align-items:center;gap:9px;padding:9px 14px;width:100%;border:none;background:none;cursor:pointer;font-family:'Inter',sans-serif;font-size:12.5px;font-weight:500;color:var(--muted);text-align:left;transition:all .12s;border-left:2px solid transparent}
.nav-btn:hover{color:var(--text);background:rgba(255,255,255,.025);border-left-color:var(--border2)}
.nav-btn.active{color:var(--text);font-weight:600;background:rgba(52,211,153,0.07);border-left-color:var(--accent2)}
.nav-btn.done{color:var(--green)}
.nav-btn.done .nav-num{background:var(--green-dim);border-color:var(--green-border);color:var(--green)}
.nav-btn.done::after{content:'✓';margin-left:auto;font-size:10px;color:var(--green)}
.nav-num{width:20px;height:20px;border-radius:5px;font-size:10px;font-weight:700;background:var(--dim);border:1px solid var(--border2);color:var(--muted);display:flex;align-items:center;justify-content:center;flex-shrink:0;font-family:'JetBrains Mono',monospace;transition:all .12s}
.nav-btn.active .nav-num{background:rgba(52,211,153,0.15);border-color:rgba(52,211,153,0.35);color:var(--accent2)}
.sb-footer{padding:14px 16px;border-top:1px solid var(--border);font-size:11px;color:var(--muted)}
.sb-footer strong{color:var(--text);display:block;margin-bottom:3px}

.main{flex:1;min-width:0;padding:48px 56px 80px;max-width:900px}

.lesson{display:none}
.lesson.active{display:block;animation:fadeUp .22s ease}
@keyframes fadeUp{from{opacity:0;transform:translateY(8px)}to{opacity:1;transform:translateY(0)}}
.eyebrow{display:flex;align-items:center;gap:8px;margin-bottom:12px}
.ey-tag{font-family:'JetBrains Mono',monospace;font-size:10px;font-weight:700;letter-spacing:.1em;text-transform:uppercase;padding:3px 9px;border-radius:4px;background:rgba(52,211,153,.12);color:var(--accent2);border:1px solid rgba(52,211,153,.22)}
.ey-num{font-family:'JetBrains Mono',monospace;font-size:11px;color:var(--muted)}
.lesson-title{font-size:clamp(1.6rem,2.8vw,2.2rem);font-weight:700;line-height:1.18;letter-spacing:-.03em;margin-bottom:16px}
.lesson-title em{font-style:normal;background:var(--grad);-webkit-background-clip:text;-webkit-text-fill-color:transparent;background-clip:text}
.lesson-intro{font-size:.94rem;color:var(--body);max-width:660px;line-height:1.85;margin-bottom:38px;padding:14px 18px;background:var(--card);border:1px solid var(--border);border-left:3px solid var(--accent2);border-radius:0 8px 8px 0}

.prose{max-width:760px}
.prose p{font-size:.92rem;color:var(--body);line-height:1.85;margin-bottom:18px}
.prose h3{font-size:1rem;font-weight:700;color:var(--text);margin:32px 0 12px;letter-spacing:-.01em;display:flex;align-items:center;gap:8px}
.prose h3::before{content:'';display:block;width:3px;height:14px;background:var(--grad);border-radius:2px;flex-shrink:0}
.prose ul,.prose ol{padding-left:22px;margin-bottom:18px}
.prose li{font-size:.9rem;color:var(--body);margin-bottom:8px;line-height:1.75}
.prose strong{color:var(--text);font-weight:600}
.prose code{font-family:'JetBrains Mono',monospace;font-size:.82em;background:var(--border);color:var(--orange);padding:2px 6px;border-radius:3px}

.code-wrap{margin:20px 0;border-radius:10px;overflow:hidden;border:1px solid var(--border2)}
.code-hdr{background:#0a0c14;display:flex;align-items:center;justify-content:space-between;padding:9px 16px;border-bottom:1px solid var(--border)}
.code-fname{font-family:'JetBrains Mono',monospace;font-size:11px;color:var(--muted);display:flex;align-items:center;gap:8px}
.code-dot{width:9px;height:9px;border-radius:50%;background:var(--accent2);opacity:.7}
.copy-btn{font-size:10px;font-family:'JetBrains Mono',monospace;color:var(--muted);background:none;border:1px solid var(--border2);padding:3px 10px;border-radius:4px;cursor:pointer;transition:all .12s}
.copy-btn:hover{color:var(--text);border-color:var(--border)}
.copy-btn.copied{color:var(--green);border-color:var(--green-border)}
pre{background:#070910;padding:20px 22px;overflow-x:auto;font-family:'JetBrains Mono',monospace;font-size:.82rem;line-height:1.9;margin:0;tab-size:4}
.kw{color:#c084fc}.cl{color:#fbbf24}.fn{color:#34d399}.str{color:#fb923c}
.num{color:#f472b6}.cm{color:#374060;font-style:italic}.ann{color:#60a5fa}
.op{color:#5a6a8a}.bl{color:#38bdf8}.typ{color:#fbbf24}

.callout{display:flex;gap:14px;padding:14px 18px;border-radius:8px;margin:20px 0;font-size:.9rem;line-height:1.75;max-width:760px}
.callout-ico{font-size:17px;flex-shrink:0;margin-top:1px}
.callout-body strong{display:block;font-size:.85rem;font-family:'JetBrains Mono',monospace;margin-bottom:4px}
.callout-body p{color:var(--muted);margin:0;font-size:.87rem;line-height:1.7}
.callout.tip{background:var(--blue-dim);border:1px solid var(--blue-border)}
.callout.tip strong{color:var(--blue)}
.callout.warn{background:var(--yellow-dim);border:1px solid var(--yellow-border)}
.callout.warn strong{color:var(--yellow)}
.callout.good{background:var(--green-dim);border:1px solid var(--green-border)}
.callout.good strong{color:var(--green)}
.callout.danger{background:var(--red-dim);border:1px solid var(--red-border)}
.callout.danger strong{color:var(--red)}

.tbl-wrap{overflow-x:auto;margin:16px 0;max-width:760px;border-radius:8px;border:1px solid var(--border)}
table{width:100%;border-collapse:collapse;font-size:.87rem}
th{text-align:left;padding:9px 14px;font-size:10px;font-weight:700;letter-spacing:.1em;text-transform:uppercase;color:var(--muted);border-bottom:1px solid var(--border2);background:var(--dim)}
td{padding:10px 14px;border-bottom:1px solid var(--border);color:var(--body);vertical-align:top}
td:first-child{font-family:'JetBrains Mono',monospace;font-size:.82rem;color:var(--text);font-weight:500}
tr:last-child td{border-bottom:none}
tr:hover td{background:rgba(52,211,153,0.03)}

.quiz{background:var(--card);border:1px solid var(--border2);border-radius:10px;padding:24px;margin:30px 0;max-width:760px}
.quiz-tag{font-family:'JetBrains Mono',monospace;font-size:9px;font-weight:700;letter-spacing:.14em;text-transform:uppercase;color:var(--accent2);margin-bottom:12px;display:flex;align-items:center;gap:6px}
.quiz-tag::before{content:'';width:3px;height:10px;background:var(--accent2);border-radius:2px;display:block}
.quiz-q{font-size:.93rem;font-weight:600;color:var(--text);margin-bottom:18px;line-height:1.55}
.quiz-opts{display:flex;flex-direction:column;gap:8px}
.quiz-opt{background:var(--surface);border:1px solid var(--border2);border-radius:7px;padding:11px 16px;cursor:pointer;font-family:'Inter',sans-serif;font-size:.88rem;color:var(--muted);text-align:left;transition:all .13s;display:flex;align-items:center;gap:10px}
.quiz-opt::before{content:'';width:16px;height:16px;border-radius:50%;border:2px solid var(--border2);flex-shrink:0;transition:all .13s}
.quiz-opt:hover:not(:disabled){color:var(--text);background:rgba(52,211,153,0.05);border-color:rgba(52,211,153,0.25)}
.quiz-opt:hover:not(:disabled)::before{border-color:var(--accent2)}
.quiz-opt.correct{border-color:var(--green);color:var(--green);background:var(--green-dim);cursor:default}
.quiz-opt.correct::before{background:var(--green);border-color:var(--green)}
.quiz-opt.wrong{border-color:var(--red);color:var(--red);background:var(--red-dim);cursor:default}
.quiz-opt.wrong::before{background:var(--red);border-color:var(--red)}
.quiz-opt:disabled{cursor:default;opacity:.7}
.quiz-fb{margin-top:14px;padding:11px 16px;border-radius:7px;font-size:.86rem;font-weight:600;font-family:'JetBrains Mono',monospace;display:none}
.quiz-fb.show{display:block}
.quiz-fb.ok{background:var(--green-dim);color:var(--green);border:1px solid var(--green-border)}
.quiz-fb.bad{background:var(--red-dim);color:var(--red);border:1px solid var(--red-border)}

.challenge{border-radius:10px;padding:22px 24px;margin:24px 0;max-width:760px;border:1px solid;position:relative;overflow:hidden}
.challenge::after{content:'';position:absolute;top:0;left:0;bottom:0;width:3px}
.ch-tag{font-family:'JetBrains Mono',monospace;font-size:9px;font-weight:700;letter-spacing:.14em;text-transform:uppercase;margin-bottom:12px}
.challenge h4{font-size:.95rem;font-weight:700;color:var(--text);margin-bottom:9px}
.challenge p{font-size:.89rem;color:var(--body);line-height:1.75;margin-bottom:10px}
.challenge ul{padding-left:18px}
.challenge li{font-size:.87rem;color:var(--body);margin-bottom:6px;line-height:1.65}
.challenge.hard{background:rgba(248,113,113,0.05);border-color:rgba(248,113,113,0.22)}
.challenge.hard::after{background:var(--accent5)}
.challenge.hard .ch-tag{color:var(--accent5)}
.challenge.med{background:rgba(245,158,11,0.05);border-color:rgba(245,158,11,0.22)}
.challenge.med::after{background:var(--accent3)}
.challenge.med .ch-tag{color:var(--accent3)}
.challenge.easy{background:rgba(52,211,153,0.05);border-color:rgba(52,211,153,0.22)}
.challenge.easy::after{background:var(--accent2)}
.challenge.easy .ch-tag{color:var(--accent2)}

.ex-label{display:inline-flex;align-items:center;gap:7px;font-family:'JetBrains Mono',monospace;font-size:10px;font-weight:700;letter-spacing:.08em;text-transform:uppercase;color:var(--accent2);margin-bottom:6px;padding:3px 10px;background:rgba(52,211,153,0.08);border:1px solid rgba(52,211,153,0.2);border-radius:5px}

.concept-grid{display:grid;grid-template-columns:repeat(auto-fill,minmax(200px,1fr));gap:12px;margin:20px 0;max-width:760px}
.concept-card{background:var(--card);border:1px solid var(--border);border-radius:8px;padding:16px;transition:border-color .15s}
.concept-card:hover{border-color:var(--border2)}
.cc-icon{font-size:20px;margin-bottom:8px}
.cc-title{font-size:.87rem;font-weight:700;color:var(--text);margin-bottom:5px}
.cc-desc{font-size:.8rem;color:var(--muted);line-height:1.6}

/* Decision cards */
.decision-grid{display:grid;grid-template-columns:1fr 1fr;gap:14px;margin:20px 0;max-width:760px}
.dec-card{background:var(--card);border:1px solid var(--border);border-radius:10px;overflow:hidden}
.dec-hdr{padding:12px 16px;display:flex;align-items:center;gap:10px;border-bottom:1px solid var(--border)}
.dec-icon{font-size:20px}
.dec-name{font-weight:700;font-size:.95rem}
.dec-sub{font-size:.8rem;color:var(--muted);font-family:'JetBrains Mono',monospace}
.dec-body{padding:14px 16px}
.dec-row{display:flex;align-items:flex-start;gap:8px;margin-bottom:9px;font-size:.86rem}
.dec-row:last-child{margin-bottom:0}
.dec-ico-s{font-size:13px;flex-shrink:0;margin-top:1px}
.dec-txt{color:var(--body);line-height:1.55}

/* Memory diagram */
.mem-diag{background:#050710;border:1px solid var(--border2);border-radius:10px;padding:20px;margin:20px 0;max-width:760px}
.mem-title{font-family:'JetBrains Mono',monospace;font-size:11px;font-weight:700;color:var(--muted);margin-bottom:14px;letter-spacing:.06em;text-transform:uppercase}
.mem-row{display:flex;align-items:center;gap:10px;margin-bottom:10px}
.mem-label{font-family:'JetBrains Mono',monospace;font-size:11px;color:var(--muted);width:120px;flex-shrink:0}
.mem-cells{display:flex;gap:2px;flex-wrap:wrap}
.mem-cell{width:36px;height:36px;border-radius:4px;display:flex;align-items:center;justify-content:center;font-family:'JetBrains Mono',monospace;font-size:11px;font-weight:700;border:1px solid}
.mc-arr{background:rgba(96,165,250,.1);border-color:rgba(96,165,250,.3);color:var(--accent4)}
.mc-al{background:rgba(52,211,153,.08);border-color:rgba(52,211,153,.25);color:var(--accent2)}
.mc-null{background:var(--dim);border-color:var(--border2);color:var(--muted);font-size:9px}
.mem-note{font-size:.8rem;color:var(--muted);margin-left:4px}

/* Utility methods table */
.method-grid{display:flex;flex-direction:column;gap:6px;margin:16px 0;max-width:760px}
.method-row{background:var(--card);border:1px solid var(--border);border-radius:7px;padding:11px 16px;display:grid;grid-template-columns:240px 1fr;gap:12px;font-size:.86rem}
.mr-code{font-family:'JetBrains Mono',monospace;color:var(--accent2);font-weight:600;font-size:.82rem}
.mr-desc{color:var(--body);line-height:1.55}

/* Comparison callout */
.compare-grid{display:grid;grid-template-columns:1fr 1fr;gap:12px;margin:20px 0;max-width:760px}
.cmp-box{background:var(--card);border:1px solid var(--border);border-radius:8px;overflow:hidden}
.cmp-hdr{padding:10px 14px;font-family:'JetBrains Mono',monospace;font-size:11px;font-weight:700;letter-spacing:.04em;border-bottom:1px solid var(--border)}
.cmp-body{padding:14px}
.cmp-body p{font-size:.85rem;color:var(--body);margin-bottom:6px;line-height:1.6}
.cmp-body code{font-family:'JetBrains Mono',monospace;font-size:.8em;background:var(--border);color:var(--orange);padding:1px 5px;border-radius:3px}

/* Cheatsheet */
.cheatsheet{background:#050710;border:1px solid var(--border2);border-radius:10px;padding:20px 24px;margin:20px 0;max-width:760px;font-family:'JetBrains Mono',monospace;font-size:.82rem;line-height:1.85}
.cs-section{font-size:9px;font-weight:700;letter-spacing:.12em;text-transform:uppercase;color:var(--muted);padding:8px 0 4px;border-bottom:1px solid var(--border);margin-bottom:8px;margin-top:12px}
.cs-section:first-child{margin-top:0}
.cs-row{display:grid;grid-template-columns:200px 1fr;gap:16px;padding:5px 0;border-bottom:1px solid rgba(31,37,53,0.5)}
.cs-row:last-child{border-bottom:none}
.cs-key{color:var(--accent4);font-weight:600}
.cs-val{color:var(--body)}

/* Practice */
.practice-list{display:flex;flex-direction:column;gap:10px;margin:20px 0;max-width:760px}
.practice-item{background:var(--card);border:1px solid var(--border);border-radius:8px;padding:14px 18px;display:flex;align-items:flex-start;gap:14px;transition:border-color .15s}
.practice-item:hover{border-color:var(--border2)}
.pi-num{width:28px;height:28px;border-radius:6px;background:rgba(52,211,153,.1);border:1px solid rgba(52,211,153,.2);display:flex;align-items:center;justify-content:center;font-family:'JetBrains Mono',monospace;font-size:11px;font-weight:700;color:var(--accent2);flex-shrink:0;margin-top:1px}
.pi-body{flex:1}
.pi-title{font-size:.9rem;font-weight:600;color:var(--text);margin-bottom:4px}
.pi-meta{font-size:.82rem;color:var(--muted);display:flex;gap:10px;flex-wrap:wrap}
.pi-tag{background:var(--dim);padding:2px 8px;border-radius:4px;font-family:'JetBrains Mono',monospace;font-size:10px}
.pi-time{color:var(--muted);font-size:11px}

.lesson-nav{display:flex;justify-content:space-between;align-items:center;margin-top:56px;padding-top:24px;border-top:1px solid var(--border);max-width:760px}
.btn{font-family:'Inter',sans-serif;font-size:12px;font-weight:700;padding:10px 22px;border-radius:6px;cursor:pointer;border:none;transition:all .15s;letter-spacing:.02em}
.btn-prev{background:var(--card);color:var(--muted);border:1px solid var(--border2)}
.btn-prev:hover:not(:disabled){color:var(--text);border-color:var(--border)}
.btn-next{background:var(--grad);color:#fff}
.btn-next:hover:not(:disabled){filter:brightness(1.1);transform:translateY(-1px);box-shadow:0 4px 20px rgba(16,185,129,.25)}
.btn:disabled{opacity:.25;cursor:not-allowed;transform:none!important;box-shadow:none!important}
.nav-indicator{font-family:'JetBrains Mono',monospace;font-size:11px;color:var(--muted)}

::-webkit-scrollbar{width:4px;height:4px}
::-webkit-scrollbar-thumb{background:var(--border2);border-radius:2px}
::-webkit-scrollbar-track{background:transparent}

@media(max-width:860px){
  .sidebar{display:none}
  .main{padding:28px 24px 60px}
  header{padding:0 20px}
  .decision-grid,.compare-grid{grid-template-columns:1fr}
}
/* Mobile Responsive Overrides */
@media (max-width: 768px) {
    .shell, .layout { 
        flex-direction: column !important; 
    }
    .sidebar { 
        width: 100% !important; 
        position: static !important;
        margin-bottom: 20px;
    }
    .main { 
        width: 100% !important; 
        padding: 15px !important;
    }
    .lesson-nav { 
        flex-direction: column; 
        gap: 10px; 
    }
    button, .nav-btn, .btn { 
        min-height: 44px !important; 
        padding: 12px !important;
    }
}