:root {
    --primary-color: #2c3e50;
    --accent-color: #3498db;
    --bg-light: #f9f9f9;
    --border-color: #ddd;
    --success: #27ae60;
}

/* 1. RESET & BASE LAYOUT */
body { 
    font-family: 'Segoe UI', Roboto, Helvetica, Arial, sans-serif; 
    line-height: 1.7; 
    color: #333; 
    margin: 0; /* Important: removes white gaps around the nav */
    padding: 0;
    background-color: #fff;
}

/* 2. NAVIGATION (Full Width) */
.main-nav {
    background: var(--primary-color);
    width: 100%;
    padding: 15px 0;
    position: sticky;
    top: 0;
    z-index: 1000;
    box-shadow: 0 4px 12px rgba(0,0,0,0.15);
}

.nav-container {
    max-width: 1100px;
    margin: 0 auto;
    padding: 0 20px;
    display: flex;
    flex-direction: column;
    align-items: center;
    gap: 15px;
}

.nav-logo {
    color: #ffffff;
    text-decoration: none;
    font-weight: 800;
    font-size: 1.4rem;
    text-transform: uppercase;
    letter-spacing: 1px;
}

.nav-links {
    display: flex;
    flex-wrap: wrap;
    justify-content: center;
    gap: 10px 18px;
}

.nav-links a {
    color: #bdc3c7;
    text-decoration: none;
    font-size: 0.85rem;
    font-weight: 500;
    transition: all 0.3s ease;
    padding: 4px 8px;
}

.nav-links a:hover {
    color: #ffffff;
}

/* 3. CONTENT WRAPPER (Centered) */
/* This keeps your math text from hitting the edges of the screen */
h1, h2, h3, section, .intro-box, .rule-card, details, .cta-box {
    max-width: 900px;
    margin-left: auto;
    margin-right: auto;
    padding-left: 20px;
    padding-right: 20px;
}

h1 { 
    margin-top: 40px;
    color: var(--primary-color); 
    border-bottom: 3px solid var(--accent-color); 
    padding-bottom: 10px; 
}

h2, h3 { color: var(--primary-color); margin-top: 30px; }

/* 4. MATH COMPONENTS */
.intro-box { 
    background: var(--bg-light); 
    border-left: 5px solid var(--accent-color); 
    padding: 20px; 
    margin: 20px auto; 
    border-radius: 0 5px 5px 0;
}

.rule-card { 
    background: #eef7ff; 
    border: 1px solid #d0e7ff; 
    border-radius: 8px; 
    padding: 20px; 
    margin: 30px auto;
}

.rule-table { width: 100%; border-collapse: collapse; margin-top: 10px; }
.rule-table td { padding: 12px; border-bottom: 1px solid #d0e7ff; }

details { 
    background: #fff; 
    border: 1px solid var(--border-color); 
    border-radius: 6px; 
    margin-bottom: 15px; 
}

summary { 
    padding: 15px; 
    font-weight: 600; 
    cursor: pointer; 
    color: var(--primary-color);
}

.solution-content { padding: 0 20px 20px 20px; border-top: 1px solid #eee; }

img { max-width: 100%; height: auto; display: block; margin: 20px auto; }
.katex-display { overflow-x: auto; padding: 10px 0; }

.cta-box {
    background: var(--primary-color);
    color: white;
    padding: 25px;
    border-radius: 10px;
    margin: 40px auto;
    text-align: center;
}
.cta-box a { color: #5dade2; text-decoration: none; font-weight: bold; }

/* 5. RESPONSIVE MEDIA QUERIES */
@media (min-width: 900px) {
    .nav-container { flex-direction: row; justify-content: space-between; }
    .nav-links { justify-content: flex-end; }
}


/* Homepage Hub Layout */
.hub-grid {
    display: grid;
    grid-template-columns: repeat(auto-fit, minmax(320px, 1fr));
    gap: 25px;
    margin: 30px auto;
    max-width: 1200px;
}

.hub-card {
    background: #fff;
    border: 1px solid var(--border-color);
    border-radius: 8px;
    padding: 20px;
    transition: transform 0.2s, box-shadow 0.2s;
    display: flex;
    flex-direction: column;
}

.hub-card:hover {
    transform: translateY(-3px);
    box-shadow: 0 5px 15px rgba(0,0,0,0.08);
    border-color: var(--accent-color);
}

.hub-card h2 {
    margin: 0 0 15px 0;
    font-size: 1.15rem;
    color: var(--primary-color);
    border-bottom: 2px solid var(--accent-color);
    padding-bottom: 8px;
    text-transform: uppercase;
    letter-spacing: 0.5px;
}

.hub-card ul {
    list-style: none;
    padding: 0;
    margin: 0;
}

.hub-card li {
    margin-bottom: 12px;
    font-size: 0.95rem;
    line-height: 1.4;
}

.hub-card a {
    color: #34495e;
    text-decoration: none;
    transition: color 0.2s;
}

.hub-card a:hover {
    color: var(--accent-color);
    text-decoration: underline;
}

.badge-new {
    background: #e74c3c;
    color: white;
    font-size: 0.65rem;
    padding: 2px 6px;
    border-radius: 4px;
    font-weight: bold;
    margin-left: 5px;
    vertical-align: middle;
}

.grade-pills {
    display: flex;
    flex-wrap: wrap;
    gap: 5px;
    margin-top: 10px;
}

.grade-pills a {
    background: #f1f1f1;
    padding: 2px 8px;
    border-radius: 4px;
    font-size: 0.8rem;
    color: var(--primary-color) !important;
}