:root,
html[data-theme="dark"] {
    --bg: #09090b;
    --bg-elevated: #111114;
    --bg-card: #16161a;
    --bg-hover: #1f1f25;
    --bg-active: #ffffff;
    --fg: #f4f4f6;
    --fg-muted: #9898a2;
    --fg-dim: #63636e;
    --fg-inverted: #09090b;
    --border: #26262c;
    --border-strong: #3f3f48;
    --border-active: #ffffff;
    --highlight-bg: #27272f;
    --highlight-border: #d4d4dc;
    --opt-correct-bg: rgba(34, 197, 94, 0.12);
    --opt-correct-border: #22c55e;
    --opt-wrong-bg: rgba(239, 68, 68, 0.12);
    --opt-wrong-border: #ef4444;
    --shadow-sm: 0 2px 8px rgba(0, 0, 0, 0.25);
    --shadow-md: 0 8px 28px rgba(0, 0, 0, 0.45);
    --radius-xs: 6px;
    --radius-sm: 8px;
    --radius-md: 12px;
    --radius-lg: 16px;
    --radius-pill: 9999px;
    --font-sans: -apple-system, BlinkMacSystemFont, "Inter", "SF Pro Text", "PingFang SC", "Hiragino Sans GB", "Microsoft YaHei", sans-serif;
    --font-mono: "JetBrains Mono", "SF Mono", "Menlo", "Consolas", monospace;
    --font-serif: "New York", "Georgia", "Songti SC", serif;
}

html[data-theme="light"] {
    --bg: #f6f6f8;
    --bg-elevated: #ffffff;
    --bg-card: #ffffff;
    --bg-hover: #efeff3;
    --bg-active: #111114;
    --fg: #111114;
    --fg-muted: #5a5a66;
    --fg-dim: #8a8a96;
    --fg-inverted: #ffffff;
    --border: #e2e2e8;
    --border-strong: #c8c8d2;
    --border-active: #111114;
    --highlight-bg: #eaeaf0;
    --highlight-border: #33333c;
    --opt-correct-bg: rgba(22, 163, 74, 0.08);
    --opt-correct-border: #16a34a;
    --opt-wrong-bg: rgba(220, 38, 38, 0.08);
    --opt-wrong-border: #dc2626;
    --shadow-sm: 0 2px 6px rgba(0, 0, 0, 0.04);
    --shadow-md: 0 10px 30px rgba(0, 0, 0, 0.08);
}

* {
    box-sizing: border-box;
    margin: 0;
    padding: 0;
}

html, body {
    background-color: var(--bg);
    color: var(--fg);
    font-family: var(--font-sans);
    font-size: 14px;
    line-height: 1.6;
    -webkit-font-smoothing: antialiased;
    min-height: 100vh;
    transition: background-color 0.18s ease, color 0.18s ease;
}

::selection {
    background: var(--fg);
    color: var(--bg);
}

::-webkit-scrollbar {
    width: 6px;
    height: 6px;
}
::-webkit-scrollbar-track {
    background: transparent;
}
::-webkit-scrollbar-thumb {
    background: var(--border-strong);
    border-radius: var(--radius-pill);
}

/* Header Navigation */
.top-header {
    position: sticky;
    top: 0;
    z-index: 100;
    display: flex;
    align-items: center;
    justify-content: space-between;
    padding: 0 24px;
    height: 60px;
    background: var(--bg-elevated);
    border-bottom: 1px solid var(--border);
    gap: 16px;
}

.brand-block {
    display: flex;
    align-items: center;
    gap: 10px;
    cursor: pointer;
    user-select: none;
}

.brand-mark {
    background: var(--fg);
    color: var(--bg);
    font-family: var(--font-mono);
    font-weight: 800;
    font-size: 13px;
    padding: 4px 10px;
    letter-spacing: 1px;
    border-radius: var(--radius-sm);
}

.brand-divider {
    color: var(--fg-dim);
    font-family: var(--font-mono);
    font-weight: 600;
}

.brand-sub {
    font-family: var(--font-mono);
    font-weight: 700;
    font-size: 13px;
    letter-spacing: 0.5px;
}

.main-nav {
    display: flex;
    align-items: center;
    gap: 4px;
    background: var(--bg);
    padding: 4px;
    border-radius: var(--radius-md);
    border: 1px solid var(--border);
}

.nav-btn {
    background: transparent;
    color: var(--fg-muted);
    border: none;
    padding: 6px 14px;
    font-family: var(--font-sans);
    font-size: 13px;
    font-weight: 500;
    cursor: pointer;
    border-radius: var(--radius-sm);
    transition: all 0.15s ease;
    display: inline-flex;
    align-items: center;
    gap: 6px;
    white-space: nowrap;
}

.nav-btn:hover {
    color: var(--fg);
    background: var(--bg-hover);
}

.nav-btn.active {
    background: var(--bg-active);
    color: var(--fg-inverted);
    font-weight: 600;
    box-shadow: var(--shadow-sm);
}

.badge {
    font-family: var(--font-mono);
    font-size: 11px;
    padding: 1px 6px;
    border-radius: var(--radius-pill);
    background: var(--bg-hover);
    color: var(--fg);
}

.nav-btn.active .badge {
    background: var(--fg-inverted);
    color: var(--bg-active);
}

.header-right {
    display: flex;
    align-items: center;
    gap: 10px;
}

.sync-status-btn {
    display: inline-flex;
    align-items: center;
    gap: 6px;
    background: var(--bg);
    color: var(--fg-muted);
    border: 1px solid var(--border);
    padding: 6px 12px;
    border-radius: var(--radius-pill);
    font-size: 12px;
    cursor: pointer;
    transition: all 0.15s ease;
    white-space: nowrap;
}

.sync-status-btn:hover {
    color: var(--fg);
    border-color: var(--border-strong);
}

.sync-dot {
    width: 7px;
    height: 7px;
    border-radius: 50%;
    background: #22c55e;
}

/* Buttons */
.btn-primary, .btn-outline, .btn-ghost {
    font-family: var(--font-sans);
    font-size: 13px;
    font-weight: 600;
    padding: 8px 16px;
    cursor: pointer;
    transition: all 0.15s ease;
    display: inline-flex;
    align-items: center;
    justify-content: center;
    gap: 6px;
    text-decoration: none;
    user-select: none;
    border-radius: var(--radius-sm);
    white-space: nowrap;
}

.btn-primary {
    background: var(--bg-active);
    color: var(--fg-inverted);
    border: 1px solid var(--border-active);
}

.btn-primary:hover {
    opacity: 0.9;
}

.btn-outline {
    background: var(--bg-card);
    color: var(--fg);
    border: 1px solid var(--border-strong);
}

.btn-outline:hover {
    background: var(--bg-hover);
    border-color: var(--fg);
}

.btn-ghost {
    background: transparent;
    color: var(--fg-muted);
    border: 1px solid var(--border);
}

.btn-ghost:hover {
    color: var(--fg);
    border-color: var(--border-strong);
    background: var(--bg-hover);
}

.btn-sm {
    padding: 5px 11px;
    font-size: 12px;
    border-radius: var(--radius-xs);
}

/* Main Container */
.main-container {
    max-width: 1400px;
    margin: 0 auto;
    padding: 24px;
}

/* Cards & Grid */
.grid-4 {
    display: grid;
    grid-template-columns: repeat(4, 1fr);
    gap: 16px;
}

.grid-2 {
    display: grid;
    grid-template-columns: repeat(2, 1fr);
    gap: 20px;
}

.grid-3 {
    display: grid;
    grid-template-columns: repeat(3, 1fr);
    gap: 16px;
}

.card {
    background: var(--bg-card);
    border: 1px solid var(--border);
    border-radius: var(--radius-md);
    padding: 20px;
    box-shadow: var(--shadow-sm);
}

.card-header {
    display: flex;
    align-items: center;
    justify-content: space-between;
    padding-bottom: 12px;
    margin-bottom: 16px;
    border-bottom: 1px solid var(--border);
}

.card-title {
    font-size: 14px;
    font-weight: 700;
}

.stat-value {
    font-family: var(--font-mono);
    font-size: 30px;
    font-weight: 700;
    line-height: 1.2;
    margin: 6px 0 4px 0;
}

.stat-label {
    font-size: 12px;
    color: var(--fg-muted);
    font-weight: 500;
}

/* Split Practice Workspace */
.workspace-layout {
    display: grid;
    grid-template-columns: 270px 1fr;
    gap: 20px;
    align-items: start;
}

.filter-panel {
    position: sticky;
    top: 80px;
    background: var(--bg-card);
    border: 1px solid var(--border);
    border-radius: var(--radius-md);
    padding: 18px;
}

.filter-group {
    margin-bottom: 16px;
}

.filter-label {
    display: block;
    font-size: 12px;
    font-weight: 600;
    color: var(--fg-muted);
    margin-bottom: 6px;
}

.select-box, .input-box, .textarea-box {
    width: 100%;
    background: var(--bg);
    color: var(--fg);
    border: 1px solid var(--border-strong);
    border-radius: var(--radius-sm);
    padding: 8px 11px;
    font-family: var(--font-sans);
    font-size: 13px;
    outline: none;
    transition: border-color 0.15s ease;
}

.select-box:focus, .input-box:focus, .textarea-box:focus {
    border-color: var(--fg);
}

/* Question Display Card */
.question-card {
    background: var(--bg-card);
    border: 1px solid var(--border);
    border-radius: var(--radius-lg);
    padding: 28px;
    box-shadow: var(--shadow-sm);
}

.question-meta-bar {
    display: flex;
    flex-wrap: wrap;
    align-items: center;
    justify-content: space-between;
    gap: 10px;
    padding-bottom: 16px;
    margin-bottom: 20px;
    border-bottom: 1px solid var(--border);
}

.tag-group {
    display: flex;
    flex-wrap: wrap;
    gap: 8px;
    align-items: center;
}

.tag {
    font-family: var(--font-mono);
    font-size: 11px;
    padding: 3px 10px;
    border: 1px solid var(--border-strong);
    border-radius: var(--radius-pill);
    background: var(--bg);
    color: var(--fg-muted);
}

.tag-solid {
    background: var(--bg-active);
    color: var(--fg-inverted);
    border-color: var(--border-active);
    font-weight: 600;
}

.tag-warning {
    border-style: dashed;
    color: var(--fg);
    background: var(--bg-hover);
}

.sample-warning-banner {
    background: var(--bg-hover);
    border: 1px solid var(--border-strong);
    border-left: 3px solid var(--fg);
    border-radius: var(--radius-sm);
    padding: 10px 14px;
    margin-bottom: 18px;
    font-size: 12.5px;
    color: var(--fg-muted);
}

.passage-box {
    font-family: var(--font-sans);
    font-size: 15.5px;
    line-height: 1.85;
    padding: 20px 22px;
    background: var(--bg);
    border: 1px solid var(--border);
    border-radius: var(--radius-md);
    margin-bottom: 20px;
    white-space: pre-wrap;
    overflow-x: auto;
}

/* KaTeX & Math overrides */
.katex {
    font-size: 1.07em !important;
}

.katex-display {
    margin: 12px 0 !important;
    overflow-x: auto;
    overflow-y: hidden;
    padding: 4px 0;
}

/* Embedded Question / Solution Figures */
.tmua-figure-box {
    margin: 14px 0;
    padding: 16px;
    background: #ffffff;
    color: #000000;
    border: 1px solid var(--border-strong);
    border-radius: var(--radius-sm);
    text-align: center;
    overflow-x: auto;
}

.tmua-figure-box img {
    max-width: 100%;
    max-height: 360px;
    height: auto;
    display: inline-block;
}

.stem-box {
    font-size: 15px;
    font-weight: 600;
    margin-bottom: 16px;
    line-height: 1.55;
}

/* Options List */
.options-list {
    display: flex;
    flex-direction: column;
    gap: 10px;
    margin-bottom: 22px;
}

.option-item {
    display: flex;
    align-items: flex-start;
    gap: 14px;
    padding: 14px 16px;
    background: var(--bg);
    border: 1px solid var(--border);
    border-radius: var(--radius-md);
    cursor: pointer;
    transition: all 0.14s ease;
    text-align: left;
    width: 100%;
    color: var(--fg);
    font-family: var(--font-sans);
    font-size: 14px;
    line-height: 1.55;
}

.option-item:hover {
    background: var(--bg-hover);
    border-color: var(--border-strong);
}

.option-key {
    font-family: var(--font-mono);
    font-weight: 700;
    font-size: 12px;
    width: 26px;
    height: 26px;
    border-radius: var(--radius-xs);
    display: inline-flex;
    align-items: center;
    justify-content: center;
    border: 1px solid var(--border-strong);
    flex-shrink: 0;
    margin-top: 1px;
}

.option-item.selected {
    border-color: var(--fg);
    background: var(--bg-hover);
}

.option-item.selected .option-key {
    background: var(--bg-active);
    color: var(--fg-inverted);
    border-color: var(--border-active);
}

.option-item.correct {
    border-color: var(--opt-correct-border);
    background: var(--opt-correct-bg);
}

.option-item.correct .option-key {
    background: var(--opt-correct-border);
    color: #ffffff;
    border-color: var(--opt-correct-border);
}

.option-item.wrong {
    border-color: var(--opt-wrong-border);
    background: var(--opt-wrong-bg);
}

.option-item.wrong .option-key {
    background: var(--opt-wrong-border);
    color: #ffffff;
    border-color: var(--opt-wrong-border);
}

/* Explanation & Annotation Boxes */
.explanation-panel {
    margin-top: 22px;
    padding: 20px;
    background: var(--bg);
    border: 1px solid var(--border-strong);
    border-radius: var(--radius-md);
}

.panel-split {
    display: grid;
    grid-template-columns: 1fr 1fr;
    gap: 16px;
    margin-top: 20px;
}

/* Question Number Matrix */
.q-matrix {
    display: grid;
    grid-template-columns: repeat(auto-fill, minmax(36px, 1fr));
    gap: 6px;
    max-height: 250px;
    overflow-y: auto;
    padding-right: 4px;
}

.q-dot {
    font-family: var(--font-mono);
    font-size: 11px;
    height: 32px;
    border-radius: var(--radius-xs);
    border: 1px solid var(--border);
    background: var(--bg);
    color: var(--fg-muted);
    cursor: pointer;
    display: flex;
    align-items: center;
    justify-content: center;
    transition: all 0.12s ease;
}

.q-dot:hover {
    border-color: var(--fg);
    color: var(--fg);
}

.q-dot.current {
    background: var(--bg-active);
    color: var(--fg-inverted);
    border-color: var(--border-active);
    font-weight: 700;
}

.q-dot.done-ok {
    border-color: var(--opt-correct-border);
    color: var(--fg);
}

.q-dot.done-err {
    border-style: dashed;
    border-color: var(--opt-wrong-border);
    color: var(--fg);
}

/* Progress Bar */
.progress-track {
    width: 100%;
    height: 6px;
    background: var(--bg);
    border-radius: var(--radius-pill);
    overflow: hidden;
    margin-top: 8px;
}

.progress-fill {
    height: 100%;
    background: var(--fg);
    border-radius: var(--radius-pill);
    transition: width 0.25s ease;
}

/* Exam Mode Header Bar */
.exam-bar {
    display: flex;
    align-items: center;
    justify-content: space-between;
    padding: 14px 20px;
    background: var(--bg-card);
    border: 1px solid var(--border-strong);
    border-radius: var(--radius-md);
    margin-bottom: 20px;
}

.timer-display {
    font-family: var(--font-mono);
    font-size: 22px;
    font-weight: 700;
    letter-spacing: 1px;
}

/* Highlighted text inside passage */
mark.user-hl {
    background: var(--highlight-bg);
    color: var(--fg);
    border-bottom: 1.5px solid var(--highlight-border);
    padding: 0 2px;
    border-radius: 3px;
}

/* Modal */
.modal-backdrop {
    position: fixed;
    inset: 0;
    background: rgba(0, 0, 0, 0.65);
    backdrop-filter: blur(3px);
    z-index: 900;
    display: flex;
    align-items: center;
    justify-content: center;
    padding: 20px;
}

.modal-card {
    background: var(--bg-card);
    border: 1px solid var(--border-strong);
    border-radius: var(--radius-lg);
    max-width: 620px;
    width: 100%;
    padding: 24px;
    box-shadow: var(--shadow-md);
}

/* Responsive */
@media (max-width: 1024px) {
    .grid-4 {
        grid-template-columns: repeat(2, 1fr);
    }
    .grid-3 {
        grid-template-columns: 1fr;
    }
    .workspace-layout {
        grid-template-columns: 1fr;
    }
    .filter-panel {
        position: static;
    }
    .panel-split {
        grid-template-columns: 1fr;
    }
}

@media (max-width: 640px) {
    .top-header {
        flex-wrap: wrap;
        height: auto;
        padding: 12px 16px;
    }
    .main-nav {
        width: 100%;
        overflow-x: auto;
    }
    .grid-4, .grid-2 {
        grid-template-columns: 1fr;
    }
}