:root{
    --primary: #2c3e50;
    --secondary: #16a085;
    --accent: #1abc9c;
    --bg: #f4f7fa;
    --card:#ffffff;
    --bg-soft:#dcfce7;
    --bg-box:#f7fff9;
    --text:#1f2d2b;
    --muted:#64748b;
    --border:#d7f3e3;
}
/* ------------------ Global ------------------ */
*{
    box-sizing:border-box;
}
html{
    scroll-behavior:smooth;
}
body{
    font-family:'Segoe UI', Tahoma, Geneva, Verdana, sans-serif;
    line-height:1.75;
    background:var(--bg);
    color:var(--text);
    margin:0;
    padding:30px;
}
/* ------------------ Layout ------------------ */
.container{
    max-width:820px;
    margin:auto;
    background:var(--card);
    padding:55px;
    border-radius:16px;
    box-shadow:0 14px 40px rgba(0,0,0,0.08);
}
/* ------------------ Typography ------------------ */
h1,h2,h3{
    color:var(--primary);
    margin-top:0;
}
h1{
    font-size:2.2rem;
    text-align:center;
    border-bottom:3px solid var(--secondary);
    padding-bottom:12px;
    margin-bottom:10px;
}
h2{
    font-size:1.55rem;
    margin-top:55px;
    margin-bottom:18px;
}
h3{
    font-size:1.2rem;
}
p{
    font-size:1.05rem;
}
.subtitle{
    text-align:center;
    color:var(--muted);
    margin-bottom:40px;
}
/* ------------------ Sections ------------------ */
section{
    margin-bottom:50px;
}
section:not(:last-of-type)::after{
    content:"";
    display:block;
    height:1px;
    margin-top:40px;
    background:linear-gradient(to right, transparent, #ddd, transparent);
}
/* ------------------ Lists ------------------ */
ul{
    margin-top:10px;
    padding-left:22px;
}
li{
    margin-bottom:6px;
}
/* ------------------ Blocks and Boxes ------------------ */
.math-block{
background:#fafbfd;
padding:18px;
border-radius:10px;
margin:25px 0;
border:1px solid #e8eef5;
text-align:center;
}
.insightbox{
    background:var(--bg-soft);
    border-left:6px solid var(--secondary);
    padding:22px;
    margin:30px 0;
    border-radius:0 10px 10px 0;
    font-style:italic;
}
.model-box{
    background:var(--bg-box);
    padding:25px;
    border-radius:12px;
    margin:30px 0;
    border:1px solid var(--border);
}
.concept-box{
    background:var(--bg-soft);
    border-left:4px solid var(--secondary);
    padding:16px 20px;
    border-radius:8px;
    margin:20px 0;
}
.examplebox{
    background:var(--bg-box);
    border-left:5px solid var(--secondary);
    padding:20px 25px;
    border-radius:10px;
    margin:20px 0;
}
.examplebox h3{
    margin-top:0;
}
.examplebox hr{
    margin:18px 0;
    border:none;
    height:1px;
    background:#ddd;
}
/* ------------------ Navigation ------------------ */
.top-nav{
    margin-bottom:35px;
}
.back-btn{
    display:inline-flex;
    align-items:center;
    gap:8px;
    padding:8px 14px;
    background:var(--bg-soft);
    border-radius:8px;
    font-weight:600;
    text-decoration:none;
    color:var(--primary);
    transition:all .25s ease;
}
.back-btn:hover{
    background:#eaf2fb;
    transform:translateX(-3px);
}
/* ------------------ Chapter Links ------------------ */
.chapter-list{
    margin-top:25px;
}
.chapter-list a{
    display:block;
    padding:18px 20px;
    margin-bottom:14px;
    background:var(--bg-box);
    border-left:5px solid var(--secondary);
    border-radius:10px;
    text-decoration:none;
    color:var(--primary);
    font-weight:500;
    transition:all .25s ease;
    position:relative;
}
.chapter-list a::after{
    content:"→";
    position:absolute;
    right:20px;
    opacity:.35;
    transition:all .25s ease;
}
.chapter-list a:hover{
    background:var(--bg-soft);
    transform:translateY(-3px);
    box-shadow:0 10px 22px rgba(0,0,0,0.08);
}
.chapter-list a:hover::after{
    opacity:1;
    right:15px;
}
/* ------------------ Links ------------------ */
a{
    color:var(--secondary);
}
/* ------------------ Divider ------------------ */
hr{
    margin:60px 0;
    border:none;
    height:1px;
    background:linear-gradient(to right, transparent, #ddd, transparent);
}
/* ------------------ Footer ------------------ */
footer{
    text-align:center;
    margin-top:60px;
    color:#7f8c8d;
    font-size:0.9rem;
}

/* ------------------ Responsive ------------------ */
@media (max-width:700px){
    body{
        padding:15px;
    }
    .container{
        padding:30px;
    }
    h1{
        font-size:1.8rem;
    }
    h2{
        font-size:1.35rem;
    }
}
/* ------------------ Diagrams ------------------ */
.diagram {
text-align: center;
}
svg {
background: white;
border-radius: 10px;
box-shadow: 0 4px 12px rgba(0, 0, 0, 0.08);
padding: 20px;
}
/* ------------------ Tables ------------------ */
table{
    width:100%;
    border-collapse:collapse;
    margin:25px 0;
    font-size:0.98rem;
    background:var(--bg-box);
    border-radius:10px;
    overflow:hidden;
    border:1px solid var(--border);
}
thead{
    background:var(--bg-soft);
}
th{
    text-align:left;
    padding:14px 16px;
    font-weight:600;
    color:var(--primary);
    border-bottom:2px solid var(--border);
}
td{
    padding:14px 16px;
    border-bottom:1px solid var(--border);
}
tbody tr:last-child td{
    border-bottom:none;
}
tbody tr:hover{
    background:#fff5f5;
    transition:background 0.2s ease;
}