:root {
    --bg-color: #eef5fb;
    --panel-bg: #ffffff;
    --card-bg: #f9fbff;
    --text-main: #112b42;
    --text-muted: #475569;
    --primary: #1f61d3;
    --primary-soft: #4f7bfd;
    --accent: #0eabff;
    --success: #1f9d55;
    --success-bg: rgba(31, 157, 85, 0.12);
    --warning: #c2780e;
    --warning-bg: rgba(194, 120, 14, 0.12);
    --danger: #d1323a;
    --border: #d8e3ef;
    --shadow: 0 18px 50px rgba(17, 43, 66, 0.08);
    --radius: 22px;
}

body {
    margin: 0;
    min-height: 100vh;
    font-family: Inter, ui-sans-serif, system-ui, -apple-system, BlinkMacSystemFont, "Segoe UI", sans-serif;
    background: radial-gradient(circle at top left, rgba(31, 97, 211, 0.12), transparent 26%),
                radial-gradient(circle at bottom right, rgba(14, 171, 255, 0.08), transparent 22%),
                var(--bg-color);
    color: var(--text-main);
    line-height: 1.7;
    font-size: 16px;
}

body.dark-mode {
    --bg-color: #07101c;
    --panel-bg: #0d1723;
    --card-bg: #111d2d;
    --text-main: #f5f8ff;
    --text-muted: #94a3b8;
    --primary: #7da6ff;
    --primary-soft: #9ec5ff;
    --accent: #8bdcff;
    --success: #6dd391;
    --success-bg: rgba(39, 187, 118, 0.16);
    --warning: #f3b450;
    --warning-bg: rgba(243, 180, 80, 0.14);
    --danger: #ff7373;
    --border: rgba(255,255,255,0.08);
    --shadow: 0 18px 50px rgba(0, 0, 0, 0.35);
}

* {
    box-sizing: border-box;
}

a {
    color: inherit;
    text-decoration: none;
}

img,
svg {
    max-width: 100%;
}

button,
input,
textarea {
    font: inherit;
}

.page-container {
    max-width: 1180px;
    margin: 0 auto;
    padding: 0 20px 30px;
}

.master-header {
    background: linear-gradient(180deg, rgba(255,255,255,0.95), rgba(255,255,255,0.72));
    border: 1px solid rgba(31, 97, 211, 0.15);
    border-radius: 30px;
    padding: 28px 32px;
    margin: 20px 0 28px;
    box-shadow: var(--shadow);
    position: relative;
    overflow: hidden;
}

.master-header::after {
    content: "";
    position: absolute;
    inset: 0;
    background: radial-gradient(circle at top left, rgba(31, 97, 211, 0.08), transparent 28%);
    pointer-events: none;
}

.master-title {
    position: relative;
    margin: 0 0 18px;
    color: var(--primary);
    font-size: clamp(2.1rem, 3vw, 3rem);
    letter-spacing: -0.05em;
}

.master-subtitle {
    position: relative;
    margin: 0;
    font-size: 0.98rem;
    line-height: 1.75;
    color: var(--text-muted);
    max-width: 780px;
}

.meta-grid {
    display: grid;
    grid-template-columns: repeat(auto-fit, minmax(230px, 1fr));
    gap: 16px;
    margin-top: 22px;
}

.meta-item {
    position: relative;
    background: var(--card-bg);
    border: 1px solid var(--border);
    border-radius: 22px;
    padding: 18px 20px;
    box-shadow: inset 0 1px 0 rgba(255,255,255,0.6);
}

.meta-label {
    display: block;
    font-size: 0.76rem;
    letter-spacing: 0.12em;
    text-transform: uppercase;
    color: var(--text-muted);
    margin-bottom: 8px;
}

.meta-value {
    color: var(--text-main);
    font-weight: 700;
    font-size: 0.98rem;
}

.theme-toggle {
    position: relative;
    display: inline-flex;
    align-items: center;
    justify-content: center;
    gap: 10px;
    margin-top: 20px;
    padding: 13px 22px;
    border-radius: 999px;
    border: none;
    color: #ffffff;
    background: var(--primary);
    cursor: pointer;
    font-weight: 700;
    letter-spacing: 0.01em;
    box-shadow: 0 16px 35px rgba(31, 97, 211, 0.14);
    transition: transform 0.25s ease, background 0.25s ease, color 0.25s ease;
}

.theme-toggle:hover {
    transform: translateY(-1px);
}

body.dark-mode .theme-toggle {
    color: var(--primary);
    background: rgba(255,255,255,0.08);
    border: 1px solid rgba(255,255,255,0.12);
}

.site-nav,
.footer-nav {
    display: flex;
    flex-wrap: wrap;
    gap: 12px;
    justify-content: center;
    align-items: center;
    margin: 0 -6px 24px;
    padding: 16px;
    background: var(--panel-bg);
    border: 1px solid var(--border);
    border-radius: 24px;
    box-shadow: var(--shadow);
}

.site-nav a,
.footer-nav a {
    display: inline-flex;
    align-items: center;
    justify-content: center;
    min-width: 140px;
    padding: 12px 18px;
    border-radius: 999px;
    border: 1px solid transparent;
    background: var(--card-bg);
    color: var(--text-main);
    font-weight: 700;
    transition: transform 0.18s ease, background 0.18s ease, color 0.18s ease, border-color 0.18s ease;
}

.site-nav a:hover,
.footer-nav a:hover {
    transform: translateY(-1px);
    background: var(--primary);
    color: #ffffff;
}

.site-nav a.active,
.footer-nav a.active {
    background: var(--primary);
    color: #ffffff;
    border-color: var(--primary);
}

.page-intro {
    background: rgba(255,255,255,0.88);
    border: 1px solid rgba(31, 97, 211, 0.12);
    border-radius: 26px;
    padding: 24px 30px;
    margin-bottom: 28px;
    box-shadow: var(--shadow);
    line-height: 1.9;
}

.card {
    background: var(--card-bg);
    border: 1px solid var(--border);
    border-radius: 24px;
    padding: 24px;
    margin-bottom: 24px;
    box-shadow: var(--shadow);
    overflow: hidden;
    transition: transform 0.25s ease, box-shadow 0.25s ease;
}

.card:hover {
    transform: translateY(-4px);
    box-shadow: 0 26px 70px rgba(17, 43, 66, 0.09);
}

.card h2,
.card h3 {
    margin-top: 0;
}

.card h2 {
    color: var(--primary);
    font-size: clamp(1.35rem, 1.65vw, 2rem);
}

.card h3 {
    color: var(--text-main);
    font-size: 1.06rem;
}

.card p,
.card li,
.card span,
.card div {
    color: var(--text-main);
}

.card ul {
    margin: 18px 0 0 20px;
    padding: 0;
}

.card li {
    margin-bottom: 12px;
}

.highlight-box,
.info-box,
.note-box,
.final-answer {
    background: rgba(31, 97, 211, 0.06);
    border: 1px solid rgba(31, 97, 211, 0.16);
    border-radius: 18px;
    padding: 20px 22px;
    margin-top: 22px;
}

.final-answer {
    background: var(--success-bg);
    border-color: rgba(31, 157, 85, 0.18);
    color: var(--success);
}

.step {
    margin-bottom: 24px;
    padding: 20px;
    background: var(--panel-bg);
    border: 1px solid var(--border);
    border-radius: 22px;
}

.step-title {
    color: var(--primary);
    font-weight: 800;
    margin-bottom: 12px;
    font-size: 1rem;
}

.step-desc {
    margin-bottom: 12px;
    font-size: 0.98rem;
}

.math-block,
.math-block-sm {
    display: block;
    width: 100%;
    overflow-x: auto;
    padding: 14px 16px;
    border-radius: 16px;
    background: rgba(31, 97, 211, 0.04);
    border: 1px solid rgba(31, 97, 211, 0.12);
    font-family: ui-monospace, SFMono-Regular, Menlo, Monaco, Consolas, "Liberation Mono", "Courier New", monospace;
    white-space: pre-wrap;
    word-break: break-word;
    text-align: left;
    margin: 14px 0;
}

.math-inline {
    color: var(--accent);
    font-weight: 700;
}

.mcq-container,
.pyq-accordion {
    border-radius: 24px;
    overflow: hidden;
}

.mcq-container {
    border: 1px solid var(--border);
    background: var(--panel-bg);
    box-shadow: var(--shadow);
}

.mcq-question {
    padding: 20px 22px;
    background: rgba(31, 97, 211, 0.06);
    border-bottom: 1px solid var(--border);
    font-weight: 700;
}

.mcq-options {
    display: grid;
    gap: 12px;
    padding: 18px 20px 22px;
}

@media (min-width: 760px) {
    .mcq-options { grid-template-columns: repeat(2, minmax(0, 1fr)); }
}

.mcq-option {
    background: var(--card-bg);
    padding: 14px 16px;
    border-radius: 16px;
    border: 1px solid var(--border);
    cursor: pointer;
    font-weight: 700;
    transition: transform 0.18s ease, border-color 0.18s ease, background 0.18s ease;
}

.mcq-option:hover {
    transform: translateY(-1px);
    border-color: var(--primary);
}

.correct-reveal {
    background: rgba(57, 188, 125, 0.12) !important;
    border-color: rgba(57, 188, 125, 0.25) !important;
    color: var(--success) !important;
}

.wrong-reveal {
    background: rgba(209, 50, 58, 0.12) !important;
    border-color: rgba(209, 50, 58, 0.2) !important;
    color: var(--danger) !important;
    opacity: 0.85;
}

.mcq-reason {
    display: none;
    padding: 18px 22px;
    background: var(--warning-bg);
    border-left: 4px solid var(--warning);
    border-radius: 0 0 22px 22px;
    color: var(--text-main);
}

.pyq-btn {
    width: 100%;
    border: none;
    background: var(--panel-bg);
    padding: 24px 26px;
    text-align: left;
    cursor: pointer;
    display: flex;
    flex-direction: column;
    gap: 14px;
    transition: background 0.2s ease;
}

.pyq-btn:hover,
.pyq-btn.active {
    background: rgba(31, 97, 211, 0.07);
}

.pyq-header-row {
    display: flex;
    justify-content: space-between;
    align-items: center;
    gap: 12px;
}

.pyq-title {
    font-size: 1rem;
    line-height: 1.5;
    margin: 0;
}

.icon {
    font-size: 1.35rem;
    color: var(--primary);
    transition: transform 0.3s ease;
}

.pyq-btn.active .icon {
    transform: rotate(45deg);
}

.solution-panel {
    max-height: 0;
    overflow: hidden;
    transition: max-height 0.45s ease;
}

.solution-content {
    padding: 0 0 22px;
}

.unit-btn {
    background: var(--panel-bg);
    border: 1px solid var(--border);
    color: var(--text-main);
    padding: 14px 20px;
    border-radius: 999px;
    font-weight: 700;
    cursor: pointer;
    box-shadow: var(--shadow);
    transition: transform 0.18s ease, background 0.18s ease, border-color 0.18s ease;
}

.unit-btn:hover,
.unit-btn.active {
    transform: translateY(-1px);
    background: var(--primary);
    color: #ffffff;
    border-color: var(--primary);
}

.unit-content { display: none; }
.unit-content.active { display: block; }

.footer-nav {
    margin-top: 36px;
    padding: 18px 22px;
}

.footer-note {
    margin: 0 auto;
    max-width: 880px;
    text-align: center;
    color: var(--text-muted);
    padding: 16px 0;
}

@media (max-width: 760px) {
    .master-header {
        padding: 22px 20px;
    }
    .theme-toggle {
        width: 100%;
    }
    .page-intro {
        padding: 20px;
    }
    .card {
        padding: 20px;
    }
    .mcq-options {
        grid-template-columns: 1fr;
    }
}