:root {
    /* Colors */
    --primary: #2563eb;
    --primary-hover: #1d4ed8;
    --bg-main: #f8fafc;
    --bg-card: #ffffff;
    --text-main: #0f172a;
    --text-muted: #64748b;
    --border: #e2e8f0;
    --shadow: 0 4px 6px -1px rgb(0 0 0 / 0.1), 0 2px 4px -2px rgb(0 0 0 / 0.1);
    --shadow-sm: 0 1px 2px 0 rgb(0 0 0 / 0.05);
    
    /* Spacing */
    --sidebar-width: 320px;
    --header-height: 64px;
}

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

body {
    font-family: 'Inter', sans-serif;
    background-color: var(--bg-main);
    color: var(--text-main);
    line-height: 1.5;
    -webkit-font-smoothing: antialiased;
}

/* App Layout */
.app-container {
    display: flex;
    min-height: 100vh;
}

/* Sidebar */
.sidebar {
    width: var(--sidebar-width);
    height: 100vh;
    background: var(--bg-card);
    border-right: 1px solid var(--border);
    display: flex;
    flex-direction: column;
    position: sticky;
    top: 0;
    z-index: 100;
    transition: transform 0.3s ease;
}

.sidebar-header {
    padding: 1.5rem;
    border-bottom: 1px solid var(--border);
}

.sidebar-header h1 {
    font-size: 1.25rem;
    font-weight: 700;
    color: var(--primary);
}

.search-box {
    padding: 1rem 1.5rem;
}

.search-box input {
    width: 100%;
    padding: 0.75rem 1rem;
    border: 1px solid var(--border);
    border-radius: 0.5rem;
    font-size: 0.875rem;
    transition: border-color 0.2s, box-shadow 0.2s;
}

.search-box input:focus {
    outline: none;
    border-color: var(--primary);
    box-shadow: 0 0 0 3px rgba(37, 99, 235, 0.1);
}

.chapter-list {
    flex: 1;
    overflow-y: auto;
    padding: 0.5rem 1rem;
}

.chapter-item {
    width: 100%;
    text-align: left;
    padding: 0.75rem 1rem;
    margin-bottom: 0.25rem;
    border: none;
    background: transparent;
    border-radius: 0.5rem;
    font-size: 0.875rem;
    font-weight: 500;
    color: var(--text-muted);
    cursor: pointer;
    transition: all 0.2s;
}

.chapter-item:hover {
    background: var(--bg-main);
    color: var(--primary);
}

.chapter-item.active {
    background: rgba(37, 99, 235, 0.05);
    color: var(--primary);
}

.sidebar-footer {
    padding: 1.5rem;
    border-top: 1px solid var(--border);
}

/* Main Content */
.main-content {
    flex: 1;
    display: flex;
    flex-direction: column;
    min-width: 0;
}

.top-bar {
    height: var(--header-height);
    background: var(--bg-card);
    border-bottom: 1px solid var(--border);
    padding: 0 2rem;
    display: flex;
    align-items: center;
    justify-content: space-between;
    position: sticky;
    top: 0;
    z-index: 90;
}

.breadcrumb {
    font-weight: 500;
    color: var(--text-muted);
}

.content-area {
    padding: 2rem;
    max-width: 900px;
    margin: 0 auto;
    width: 100%;
}

/* Cards & Components */
.card {
    background: var(--bg-card);
    border-radius: 1rem;
    padding: 2rem;
    box-shadow: var(--shadow-sm);
    margin-bottom: 2rem;
    border: 1px solid var(--border);
    animation: fadeIn 0.4s ease-out;
}

.chapter-title {
    font-size: 1.875rem;
    font-weight: 700;
    margin-bottom: 1rem;
    color: var(--text-main);
}

.section-title {
    font-size: 1.25rem;
    font-weight: 600;
    margin: 2rem 0 1rem;
    display: flex;
    align-items: center;
    gap: 0.5rem;
}

/* Accordions */
.accordion {
    margin-bottom: 0.75rem;
    border: 1px solid var(--border);
    border-radius: 0.75rem;
    overflow: hidden;
}

.accordion-header {
    width: 100%;
    padding: 1rem 1.25rem;
    background: var(--bg-card);
    border: none;
    text-align: left;
    font-weight: 600;
    display: flex;
    justify-content: space-between;
    align-items: center;
    cursor: pointer;
    transition: background 0.2s;
}

.accordion-header:hover {
    background: #f1f5f9;
}

.accordion-header::after {
    content: '↓';
    font-size: 0.8rem;
    transition: transform 0.3s;
}

.accordion.open .accordion-header::after {
    transform: rotate(180deg);
}

.accordion-content {
    max-height: 0;
    overflow: hidden;
    transition: max-height 0.3s ease-out;
    background: #fdfdfd;
}

.accordion.open .accordion-content {
    max-height: 500px; /* Large enough to fit content */
    padding: 1rem 1.25rem;
    border-top: 1px solid var(--border);
}

/* Questions */
.question-item {
    background: #f8fafc;
    padding: 1.25rem;
    border-radius: 0.75rem;
    margin-bottom: 1rem;
}

.question-text {
    font-weight: 600;
    margin-bottom: 1rem;
}

.answer-box {
    margin-top: 1rem;
    padding-top: 1rem;
    border-top: 1px dashed var(--border);
}

.hidden {
    display: none;
}

/* Buttons */
.btn {
    display: inline-flex;
    align-items: center;
    justify-content: center;
    padding: 0.625rem 1.25rem;
    border-radius: 0.5rem;
    font-weight: 500;
    cursor: pointer;
    transition: all 0.2s;
    border: none;
    font-size: 0.875rem;
    gap: 0.5rem;
}

.btn-primary {
    background: var(--primary);
    color: white;
}

.btn-primary:hover {
    background: var(--primary-hover);
}

.btn-secondary {
    background: #e2e8f0;
    color: var(--text-main);
}

.btn-secondary:hover {
    background: #cbd5e1;
}

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

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

.btn-block {
    width: 100%;
}

/* Modal */
.modal {
    display: none;
    position: fixed;
    top: 0;
    left: 0;
    width: 100%;
    height: 100%;
    background: rgba(15, 23, 42, 0.5);
    backdrop-filter: blur(4px);
    z-index: 1000;
    align-items: center;
    justify-content: center;
    padding: 1rem;
}

.modal-content {
    background: var(--bg-card);
    width: 100%;
    max-width: 600px;
    border-radius: 1.5rem;
    box-shadow: 0 25px 50px -12px rgb(0 0 0 / 0.25);
    overflow: hidden;
    animation: modalIn 0.3s ease-out;
}

.modal-header {
    padding: 1.5rem;
    border-bottom: 1px solid var(--border);
    display: flex;
    justify-content: space-between;
    align-items: center;
}

.modal-body {
    padding: 2rem;
}

.quiz-question {
    font-size: 1.5rem;
    font-weight: 700;
    text-align: center;
    margin-bottom: 2rem;
}

.quiz-footer {
    padding: 1.5rem;
    background: #f8fafc;
    display: flex;
    justify-content: flex-end;
    gap: 1rem;
}

/* Utilities */
.badge {
    padding: 0.25rem 0.75rem;
    background: rgba(37, 99, 235, 0.1);
    color: var(--primary);
    border-radius: 9999px;
    font-size: 0.75rem;
    font-weight: 600;
}

.mobile-only {
    display: none;
}

.btn-icon {
    background: none;
    border: none;
    cursor: pointer;
    color: var(--text-muted);
}

@keyframes fadeIn {
    from { opacity: 0; transform: translateY(10px); }
    to { opacity: 1; transform: translateY(0); }
}

@keyframes modalIn {
    from { opacity: 0; transform: scale(0.95); }
    to { opacity: 1; transform: scale(1); }
}

/* Mobile Responsive */
@media (max-width: 768px) {
    .sidebar {
        position: fixed;
        left: 0;
        top: 0;
        transform: translateX(-100%);
    }

    .sidebar.open {
        transform: translateX(0);
    }

    .mobile-only {
        display: block;
    }

    .top-bar {
        padding: 0 1rem;
    }

    .content-area {
        padding: 1rem;
    }
}