/* ===== Import Fonts (nếu cần) ===== */
@import url('https://fonts.googleapis.com/css2?family=Segoe+UI:wght@400;600;700&display=swap');

/* ===== CSS Variables ===== */
:root {
    --primary-color: #c8102e;
    --secondary-color: #ffd700;
    --text-color: #333;
    --light-bg: #f5f5f5;
    --success: #28a745;
    --info: #17a2b8;
    --note-color: #FF9800;
    --theorem-color: #4CAF50;
}

/* ===== Reset ===== */
* {
    margin: 0;
    padding: 0;
    box-sizing: border-box;
    font-family: 'Segoe UI', Arial, sans-serif;
}

body {
    line-height: 1.6;
    color: var(--text-color);
    padding-top: 90px;
    background: var(--light-bg);
}

/* ===== Header ===== */
header {
    background-color: var(--primary-color);
    color: white;
    padding: 10px 0;
    position: fixed;
    top: 0;
    left: 0;
    width: 100%;
    z-index: 1000;
    box-shadow: 0 2px 8px rgba(0,0,0,0.2);
}

.header-container {
    display: flex;
    justify-content: space-between;
    align-items: center;
    max-width: 1200px;
    margin: 0 auto;
    padding: 0 20px;
}

.logo {
    display: flex;
    align-items: center;
    gap: 15px;
}

.logo img {
    height: 60px;
}

.logo-text h1 {
    font-size: 22px;
    font-weight: bold;
    line-height: 1.2;
    margin-bottom: 5px;
}

.logo-text p {
    font-size: 14px;
    font-weight: bold;
    color: var(--secondary-color);
    text-shadow: 0 1px 2px rgba(249, 249, 249, 0.3);
}

/* ===== Navigation ===== */
nav ul {
    display: flex;
    list-style: none;
    align-items: center;
}

nav ul li {
    margin-left: 20px;
    position: relative;
}

nav ul li a {
    color: white;
    text-decoration: none;
    font-weight: 600;
    font-size: 16px;
    transition: all 0.3s;
    padding: 8px 0;
    display: block;
    position: relative;
}

nav ul li a:hover {
    color: var(--secondary-color);
}

nav ul li a:after {
    content: '';
    position: absolute;
    width: 0;
    height: 2px;
    background: var(--secondary-color);
    bottom: 0;
    left: 0;
    transition: width 0.3s;
}

nav ul li a:hover:after {
    width: 100%;
}

.register-btn {
    background: white;
    color: var(--primary-color);
    padding: 8px 20px;
    border-radius: 30px;
    font-weight: bold;
    margin-left: 15px;
    transition: all 0.3s;
}

.register-btn:hover {
    background: #f0f0f0;
    transform: translateY(-2px);
    box-shadow: 0 4px 8px rgba(0,0,0,0.1);
}

/* ===== Mobile Menu Toggle ===== */
.menu-toggle {
    display: none;
    cursor: pointer;
    font-size: 24px;
    color: white;
    padding: 15px;
    z-index: 1001;
}

/* ===== Multi-level Dropdown Menu ===== */
nav ul li ul {
    display: none;
    position: absolute;
    top: 100%;
    left: 0;
    background: white;
    min-width: 200px;
    box-shadow: 0 5px 15px rgba(0,0,0,0.1);
    z-index: 100;
    opacity: 0;
    visibility: hidden;
    transition: opacity 0.3s, visibility 0.3s, transform 0.3s;
    transform: translateY(10px);
}

nav ul li:hover > ul {
    display: block;
    opacity: 1;
    visibility: visible;
    transform: translateY(0);
}

nav ul li ul li ul {
    top: 0;
    left: 100%;
}

nav ul li ul a {
    color: var(--text-color);
    padding: 10px 15px;
    white-space: nowrap;
    transition: all 0.3s;
}

nav ul li ul a:hover {
    background: var(--light-bg);
    color: var(--primary-color);
}

nav ul ul li {
    margin: 0;
    border-bottom: 1px solid #eee;
}

nav ul ul li:last-child {
    border-bottom: none;
}

nav ul ul .fa-chevron-right {
    float: right;
    font-size: 12px;
    margin-top: 5px;
}

/* Mobile adjustments */
@media (max-width: 768px) {
    .menu-toggle {
        display: block;
    }
    nav {
        position: fixed;
        top: 80px;
        left: -100%;
        width: 80%;
        height: calc(100vh - 80px);
        background-color: var(--primary-color);
        transition: left 0.3s ease;
        overflow-y: auto;
    }
    nav.active {
        left: 0;
    }
    nav ul {
        flex-direction: column;
        padding: 20px 0;
    }
    nav ul li {
        margin: 0;
        width: 100%;
        text-align: left;
        border-bottom: 1px solid rgba(255,255,255,0.1);
    }
    nav ul li a {
        padding: 15px 20px;
    }
    nav ul li ul {
        position: static;
        background: rgba(255,255,255,0.1);
        box-shadow: none;
        display: none;
        opacity: 1;
        visibility: visible;
        transform: none;
        width: 100%;
    }
    nav ul li.active > ul {
        display: block;
    }
    nav ul li ul li {
        border-bottom: none;
    }
    nav ul li ul a {
        padding-left: 30px;
        color: white;
    }
    nav ul li ul a:hover {
        color: var(--secondary-color);
        background: transparent;
    }
    nav ul li ul ul a {
        padding-left: 45px;
    }
    .register-btn {
        margin: 15px 20px;
        width: calc(100% - 40px);
        text-align: center;
    }
}

/* ===== Main container ===== */
h1 {
    color: #e9f501;
    text-align: center;
    font-size: 2rem;
    margin-bottom: 30px;
}

h2 {
    color: #10b981;
    font-size: 2rem;
    margin-bottom: 15px;
}

h3 {
    color: #130467;
    font-size: 1.5rem;
    margin-bottom: 10px;
}


.container {
    max-width: 1200px; /* Thay đổi từ 1200px */
    margin: 30px auto;
    padding: 0 10px; /* Giảm từ 0 20px */
    display: flex;
    gap: 30px;
}
/* ===== Left sidebar ===== */
.sidebar {
    width: 0%;
}

.sidebar-section {
    background: white;
    border-radius: 5px;
    box-shadow: 0 2px 5px rgba(0,0,0,0.1);
    margin-bottom: 20px;
    overflow: hidden;
}

.sidebar-title {
    background-color: var(--primary-color);
    color: white;
    padding: 12px 15px;
    font-size: 18px;
    font-weight: bold;
}

.article-list {
    list-style: none;
}

.article-list li {
    border-bottom: 1px solid #eee;
}

.article-list li:last-child {
    border-bottom: none;
}

.article-list a {
    display: block;
    padding: 12px 15px;
    color: #333;
    text-decoration: none;
    transition: all 0.3s;
}

.article-list a:hover {
    background-color: #f0f0f0;
    color: var(--primary-color);
    padding-left: 20px;
}

/* ===== Main content ===== */

.main-content {
    width: 100%;
    background: rgb(218, 240, 186);
    border-radius: 5px;
    box-shadow: 0 2px 5px rgba(0,0,0,0.1);
    padding: 30px;
    overflow-wrap: break-word;
    word-wrap: break-word;
    margin: 0; /* Thêm dòng này để bỏ margin hai bên */
}
.article-title {
    color: var(--primary-color);
    font-size: 28px;
    margin-bottom: 20px;
    padding-bottom: 10px;
    border-bottom: 2px solid var(--secondary-color);
    text-align: center;
}

.doc-subtitle {
    color: #555;
    font-size: 18px;
    margin-bottom: 15px;
    text-align: center;
}

.doc-meta {
    display: flex;
    justify-content: center;
    gap: 20px;
    color: #666;
    font-size: 14px;
    flex-wrap: wrap;
    margin-bottom: 30px;
}

.doc-meta i {
    color: var(--primary-color);
    margin-right: 5px;
}

.article-subtitle {
    color: var(--primary-color);
    font-size: 22px;
    padding-bottom: 8px;
    border-bottom: 1px dashed #ccc;
    margin: 30px 0 15px;
    display: flex;
    align-items: center;
}

.article-subtitle i {
    margin-right: 10px;
    font-size: 20px;
}

.theory-box {
    background: #cce7ab;
    padding: 20px;
    border-radius: 8px;
    border-left: 4px solid var(--primary-color);
    margin: 20px 0;
}

.problem-type {
    background: #fff5f5;
    padding: 20px;
    border-radius: 8px;
    margin: 20px 0;
    position: relative;
}

.problem-type:before {
    content: "Dạng toán";
    position: absolute;
    top: -10px;
    left: 15px;
    background: var(--primary-color);
    color: white;
    padding: 2px 10px;
    border-radius: 20px;
    font-size: 13px;
    font-weight: bold;
}

.problem-header {
    display: flex;
    align-items: center;
    margin-bottom: 15px;
}

.problem-number {
    width: 30px;
    height: 30px;
    background: var(--primary-color);
    color: white;
    border-radius: 50%;
    display: flex;
    align-items: center;
    justify-content: center;
    margin-right: 10px;
    font-weight: bold;
}

.example {
    background: #fff8e6;
    padding: 20px;
    border-radius: 8px;
    margin: 20px 0;
    position: relative;
}

.example:before {
    content: "Ví dụ";
    position: absolute;
    top: -10px;
    left: 15px;
    background: var(--secondary-color);
    color: #222;
    padding: 2px 10px;
    border-radius: 20px;
    font-size: 13px;
    font-weight: bold;
}

.example-header {
    display: flex;
    justify-content: space-between;
    align-items: center;
    margin-bottom: 15px;
}

.tag {
    display: inline-block;
    padding: 3px 10px;
    background: #e0e0e0;
    border-radius: 20px;
    font-size: 13px;
    margin-right: 5px;
}

.download-bar {
    display: flex;
    justify-content: space-between;
    align-items: center;
    background: #f0f0f0;
    padding: 15px;
    border-radius: 8px;
    margin-top: 30px;
}

.download-info {
    display: flex;
    align-items: center;
}

.download-info i {
    color: var(--primary-color);
    font-size: 20px;
    margin-right: 10px;
}

.formula-display {
    background: #f5f5f5;
    padding: 15px;
    border-radius: 8px;
    margin-bottom: 15px;
    overflow-x: auto;
}

.method-grid {
    display: grid;
    grid-template-columns: repeat(auto-fit, minmax(250px, 1fr));
    gap: 15px;
    margin-bottom: 15px;
}

.method-box {
    background: #e8f5e9;
    padding: 15px;
    border-radius: 8px;
    border-left: 4px solid #28a745;
}

.method-box h4 {
    margin-top: 0;
    color: #28a745;
}

.quiz-section {
    margin-bottom: 30px;
}

.quiz-item {
    background: white;
    padding: 15px;
    border-radius: 8px;
    margin-bottom: 15px;
    box-shadow: 0 2px 5px rgba(0,0,0,0.05);
}

.quiz-options {
    display: grid;
    grid-template-columns: repeat(auto-fit, minmax(200px, 1fr));
    gap: 10px;
    margin-bottom: 15px;
}

.true-false-questions {
    display: grid;
    gap: 15px;
}

.short-answer {
    display: flex;
    align-items: center;
    gap: 10px;
}

.short-answer input {
    flex: 1;
    padding: 8px 12px;
    border: 1px solid #ddd;
    border-radius: 4px;
}

.tools-grid {
    display: grid;
    grid-template-columns: repeat(auto-fit, minmax(250px, 1fr));
    gap: 15px;
}

.tool-btn {
    display: flex;
    align-items: center;
    justify-content: center;
    padding: 15px;
    text-align: left;
}

.tool-btn i {
    font-size: 24px;
    margin-right: 10px;
}

.result-message {
    margin-top: 15px;
    padding: 10px 15px;
    border-radius: 5px;
    font-weight: bold;
}

.correct {
    background: #e6ffed;
    color: #28a745;
    border-left: 4px solid #28a745;
}

.incorrect {
    background: #ffebee;
    color: #d32f2f;
    border-left: 4px solid #d32f2f;
}

.formula-box {
    margin-top: 20px;
    padding: 15px;
    background: #e3f2fd;
    border-radius: 8px;
}

.formula-box h4 {
    margin-top: 0;
    color: #0d47a1;
    display: flex;
    align-items: center;
}

.formula-box h4 i {
    margin-right: 8px;
}

.formula-grid {
    display: grid;
    grid-template-columns: repeat(auto-fit, minmax(200px, 1fr));
    gap: 10px;
    margin-top: 10px;
}

/* style phương pháp */
.solution-steps {
    background: white;
    padding: 20px;
    border-radius: 8px;
    border: 1px solid #e0e0e0;
}

.solution-steps ol {
    list-style-type: none;
    counter-reset: step-counter;
    padding-left: 0;
}

.solution-steps li {
    counter-increment: step-counter;
    position: relative;
    padding: 15px 15px 15px 85px;
    margin-bottom: 15px;
    background: #f0f7ff;
    border-radius: 8px;
    border-left: 4px solid var(--primary-color);
    transition: all 0.3s ease;
}

.solution-steps li:hover {
    background: #e0efff;
    transform: translateX(5px);
}

.solution-steps li::before {
    content: counter(step-counter);
    position: absolute;
    left: 20px;
    top: 50%;
    transform: translateY(-50%);
    background: var(--primary-color);
    color: white;
    width: 30px;
    height: 30px;
    border-radius: 50%;
    display: flex;
    align-items: center;
    justify-content: center;
    font-weight: bold;
    font-size: 16px;
}

.solution-steps li::after {
    content: "\f0a4"; /* Mã Unicode cho icon hand-point-right trong Font Awesome */
    font-family: "Font Awesome 6 Free";
    font-weight: 900;
    position: absolute;
    left: 50px;
    top: 50%;
    transform: translateY(-50%);
    color: var(--primary-color);
    font-size: 20px;
}

.solution-steps li:last-child {
    margin-bottom: 0;
}

/* ===== Footer ===== */
footer {
    background: #1a1a1a;
    color: white;
    padding: 60px 0 20px;
}

.footer-container {
    display: grid;
    grid-template-columns: repeat(3, 1fr);
    gap: 40px;
    max-width: 1200px;
    margin: 0 auto;
    padding: 0 20px;
}

.footer-col h3 {
    font-size: 20px;
    margin-bottom: 20px;
    position: relative;
    padding-bottom: 10px;
    color: var(--primary-color);
}

.footer-col h3:after {
    content: '';
    position: absolute;
    width: 50px;
    height: 2px;
    background: var(--primary-color);
    bottom: 0;
    left: 0;
}

.footer-col p {
    color: #bbb;
    font-size: 15px;
    line-height: 1.6;
}

.footer-col ul {
    list-style: none;
}

.footer-col ul li {
    margin-bottom: 10px;
}

.footer-col ul li a {
    color: #ddd;
    text-decoration: none;
    transition: all 0.3s;
    display: inline-block;
}

.footer-col ul li a:hover {
    color: var(--secondary-color);
    transform: translateX(5px);
}

.contact-info p {
    margin-bottom: 15px;
    display: flex;
    align-items: center;
    color: #ddd;
}

.contact-info i {
    margin-right: 10px;
    color: var(--primary-color);
    width: 20px;
    text-align: center;
}

.social-links a {
    display: inline-block;
    color: white;
    margin-right: 15px;
    font-size: 20px;
    transition: all 0.3s;
}

.social-links a:hover {
    color: var(--secondary-color);
    transform: translateY(-3px);
}

.copyright {
    text-align: center;
    padding-top: 30px;
    margin-top: 30px;
    border-top: 1px solid rgba(255,255,255,0.1);
    color: #bbb;
    font-size: 14px;
}

/* ===== Responsive ===== */
@media (max-width: 992px) {
    .footer-container {
        grid-template-columns: repeat(2, 1fr);
    }
}

@media (max-width: 768px) {
    .container {
        flex-direction: column;
    }
    
    .sidebar, .main-content {
        width: 100%;
    }
    
    .footer-container {
        grid-template-columns: 1fr;
    }

    .quiz-options, .method-grid, .formula-grid {
        grid-template-columns: 1fr;
    }
}

@media (max-width: 480px) {
    .doc-meta {
        flex-direction: column;
        gap: 5px;
        align-items: center;
    }
    
    .download-bar {
        flex-direction: column;
        gap: 10px;
    }
}

/* ===== Nút kiểm tra và xem lời giải ===== */
.btn {
    display: inline-block;
    background-color: var(--primary-color);
    color: white;
    padding: 10px 20px;
    border-radius: 5px;
    text-decoration: none;
    font-weight: bold;
    border: none;
    cursor: pointer;
    font-size: 14px;
    transition: all 0.3s;
    margin: 5px 0;
}

.btn:hover {
    background-color: #a50e26;
    transform: translateY(-2px);
    box-shadow: 0 2px 5px rgba(0,0,0,0.2);
}

.solution-toggle {
    background-color: var(--primary-color);
    color: white;
    border: none;
    padding: 10px 20px;
    border-radius: 5px;
    cursor: pointer;
    font-weight: bold;
    margin: 10px 0;
    display: inline-block;
}

.solution-toggle:hover {
    background-color: #a50e26;
}

.solution-toggle i {
    margin-right: 5px;
}

/* Nút kiểm tra trong quiz */
.quiz-options label {
    display: block;
    padding: 8px 12px;
    background: #f5f5f5;
    border-radius: 4px;
    margin-bottom: 5px;
    cursor: pointer;
    transition: all 0.2s;
}

.quiz-options label:hover {
    background: #e0e0e0;
}

.quiz-options input[type="radio"] {
    margin-right: 8px;
}

/* Nút kiểm tra đúng/sai */
.tf-options {
    display: flex;
    gap: 15px;
    margin-top: 5px;
}

.tf-options label {
    display: flex;
    align-items: center;
    gap: 5px;
    cursor: pointer;
}

/* 1. Style cho các loại box mới */

/* Box định nghĩa */
.definition-box {
    background: linear-gradient(135deg, #c9d0d3 0%, #f0f9eb 100%);
    border-left: 5px solid var(--theorem-color);
    border-radius: 8px;
    padding: 20px;
    margin: 25px 0;
    position: relative;
    box-shadow: 0 4px 12px rgba(0, 0, 0, 0.1);
    overflow: hidden;
}

.definition-box::before {
    content: "";
    position: absolute;
    top: 0;
    right: 0;
    width: 80px;
    height: 80px;
    background: rgba(76, 175, 80, 0.1);
    border-radius: 0 0 0 80px;
    z-index: 0;
}

.definition-icon {
    position: absolute;
    top: 20px;
    right: 20px;
    width: 40px;
    height: 40px;
    background: var(--theorem-color);
    border-radius: 50%;
    display: flex;
    align-items: center;
    justify-content: center;
    color: white;
    font-size: 18px;
    box-shadow: 0 4px 8px rgba(76, 175, 80, 0.3);
    z-index: 2;
}

.definition-content {
    position: relative;
    z-index: 1;
    padding-right: 60px;
}

.definition-content h3 {
    color: #2E7D32;
    margin-top: 0;
    margin-bottom: 12px;
    font-size: 1.4em;
    display: flex;
    align-items: center;
    border-bottom: 2px dashed #A5D6A7;
    padding-bottom: 8px;
}

.definition-content h3 i {
    margin-right: 10px;
    color: var(--theorem-color);
}

.definition-content p {
    color: #333;
    line-height: 1.6;
    margin-bottom: 0;
    font-size: 1.05em;
}

/* Hiệu ứng hover */
.definition-box:hover {
    transform: translateY(-3px);
    box-shadow: 0 8px 16px rgba(0, 0, 0, 0.15);
    transition: all 0.3s ease;
}

.definition-box:hover .definition-icon {
    transform: rotate(5deg) scale(1.05);
    transition: all 0.3s ease;
}

/* Box định lý */
.theorem-box {
    background: linear-gradient(135deg, #e6f7ff 0%, #f0f9eb 100%);
    border-left: 5px solid var(--theorem-color);
    border-radius: 10px;
    padding: 25px;
    margin: 30px 0;
    position: relative;
    box-shadow: 0 5px 15px rgba(0, 0, 0, 0.08);
}

.theorem-header {
    display: flex;
    align-items: center;
    margin-bottom: 20px;
    color: #2E7D32;
}

.theorem-icon {
    width: 40px;
    height: 40px;
    background: var(--theorem-color);
    border-radius: 50%;
    display: flex;
    align-items: center;
    justify-content: center;
    margin-right: 15px;
    color: white;
    font-size: 20px;
}

.theorem-title {
    font-size: 1.4em;
    font-weight: bold;
}

.theorem-content ul {
    list-style: none;
    padding: 0;
}

.theorem-content li {
    margin-bottom: 15px;
    padding-left: 40px;
    position: relative;
    font-size: 1.05em;
}

.theorem-content li i {
    background: transparent;
    border-radius: 0;
    width: auto;
    height: auto;
    color: var(--theorem-color);
}

/* Box chú ý */
.note-box {
    background: linear-gradient(135deg, #fff9e6 0%, #fff0f0 100%);
    border-left: 5px solid var(--note-color);
    border-radius: 10px;
    padding: 25px;
    margin: 30px 0;
    position: relative;
    box-shadow: 0 5px 15px rgba(0, 0, 0, 0.08);
}

.note-header {
    display: flex;
    align-items: center;
    margin-bottom: 20px;
    color: #E65100;
}

.note-icon {
    width: 40px;
    height: 40px;
    background: var(--note-color);
    border-radius: 50%;
    display: flex;
    align-items: center;
    justify-content: center;
    margin-right: 15px;
    color: white;
    font-size: 20px;
}

.note-title {
    font-size: 1.4em;
    font-weight: bold;
}

.note-content ul {
    list-style: none;
    padding: 0;
}

.note-content li {
    margin-bottom: 15px;
    padding-left: 40px;
    position: relative;
    font-size: 1.05em;
}

.note-content li i {
    background: transparent;
    border-radius: 0;
    width: auto;
    height: auto;
    color: var(--note-color);
}

/* Công thức toán học */
.math {
    font-family: 'Times New Roman', serif;
    font-style: italic;
    display: inline-block;
}

/* Hiệu ứng hover */
.theorem-box:hover, .note-box:hover {
    transform: translateY(-3px);
    box-shadow: 0 8px 20px rgba(0, 0, 0, 0.12);
    transition: all 0.3s ease;
}

/* Responsive for boxes */
@media (max-width: 768px) {
    .theorem-content li, .note-content li {
        padding-left: 35px;
    }
    
    .theorem-content li i, .note-content li i {
        width: 25px;
        height: 25px;
        font-size: 12px;
    }
}

/* 2. Style cho tiêu đề bên trong các box mới */
.note-box h4 {
    color: #f31212;
    margin-top: 0;
    font-size: 1.1em;
    display: flex;
    align-items: center;
}

.theorem-box h4 {
    color: #035734;
    margin-top: 0;
    font-size: 1.2em;
    display: flex;
    align-items: center;
}

.note-box h4 i, .theorem-box h4 i {
    margin-right: 8px;
}

/* Style cho phần bài tập vận dụng */
.quiz-section {
    margin-bottom: 30px;
    border: 1px solid #e0e0e0;
    border-radius: 8px;
    padding: 20px;
    background-color: #f9f9f9;
}

.quiz-item {
    margin-bottom: 20px;
    padding: 15px;
    background-color: white;
    border-radius: 8px;
    box-shadow: 0 2px 4px rgba(0,0,0,0.1);
}

.quiz-question {
    font-weight: bold;
    margin-bottom: 15px;
}

.quiz-options label {
    display: block;
    margin-bottom: 10px;
    padding: 8px;
    border-radius: 4px;
    transition: background-color 0.2s;
}

.quiz-options label:hover {
    background-color: #f0f0f0;
}

.true-false-questions {
    margin: 15px 0;
}

.tf-question {
    margin-bottom: 15px;
    padding: 10px;
    background-color: #f5f5f5;
    border-radius: 6px;
}

.tf-options {
    display: flex;
    gap: 15px;
    margin-top: 8px;
}

.tf-options label {
    display: flex;
    align-items: center;
    gap: 5px;
}

.short-answer {
    margin-top: 15px;
}

.short-answer input {
    padding: 8px 12px;
    border: 1px solid #ddd;
    border-radius: 4px;
    width: 200px;
    margin-right: 10px;
}

.result-message {
    margin-top: 10px;
    padding: 10px;
    border-radius: 4px;
    font-weight: bold;
}

.result-message.correct {
    background-color: #d4edda;
    color: #155724;
    border: 1px solid #c3e6cb;
}

.result-message.incorrect {
    background-color: #f8d7da;
    color: #721c24;
    border: 1px solid #f5c6cb;
}

.solution {
    background-color: #b8d3cc;
    color: #06011e;
    font-family: 'Courier New', 'Segoe Script', cursive;
    padding: 15px;
    margin-top: 15px;
    border-radius: 8px;
    border: 1px solid #0a2e23;
    box-shadow: 2px 2px 8px rgba(0,0,0,0.4);
    display: none;
}

.solution.active {
    display: block;
}

.solution-toggle {
    margin-top: 10px;
    padding: 8px 16px;
    background-color: #0b3a2e;
    color: white;
    border: none;
    border-radius: 4px;
    cursor: pointer;
    transition: background-color 0.3s;
}

.solution-toggle:hover {
    background-color: #145a47;
}

.option-icon {
    margin-left: 5px;
}

/* CSS cho thanh điều hướng tài liệu */
.document-nav {
    list-style: none;
}

.document-nav li {
    margin-bottom: 5px;
}

.document-nav a {
    display: block;
    padding: 10px 15px;
    color: #333;
    text-decoration: none;
    border-left: 3px solid transparent;
    transition: all 0.3s;
}

.document-nav a:hover,
.document-nav a.active {
    background: #f0f0f0;
    border-left: 3px solid #c8102e;
    color: #c8102e;
}

.document-nav .submenu {
    padding-left: 20px;
    display: none;
}

.document-nav .active .submenu {
    display: block;
}

/* Responsive */
@media (max-width: 768px) {
    .document-nav .submenu {
        padding-left: 15px;
    }
}

ul {
  list-style-type: none;
  padding-left: 0; /* Để bỏ khoảng thụt lề */
}

/* Modal styles */
.modal {
    display: none;
    position: fixed;
    top: 0;
    left: 0;
    width: 100%;
    height: 100%;
    background: rgba(0, 0, 0, 0.5);
    z-index: 1000;
    justify-content: center;
    align-items: center;
    transition: opacity 0.3s;
}

.modal.active {
    display: flex;
    opacity: 1;
}

.modal.hidden {
    display: none;
}

.modal form {
    background: white;
    padding: 20px;
    border-radius: 8px;
    box-shadow: 0 4px 8px rgba(0, 0, 0, 0.2);
    width: 300px;
    text-align: center;
}

.modal input[type="text"] {
    width: 100%;
    padding: 8px;
    margin-bottom: 10px;
    border: 1px solid #ddd;
    border-radius: 4px;
}

.modal button {
    padding: 8px 16px;
    margin: 5px;
    border: none;
    border-radius: 4px;
    cursor: pointer;
}

.modal button:nth-child(1) {
    background-color: var(--primary-color);
    color: white;
}

.modal button:nth-child(1):hover {
    background-color: #a50e26;
}

.modal button:nth-child(2) {
    background-color: #ccc;
    color: #333;
}

.modal button:nth-child(2):hover {
    background-color: #bbb;
}
/* Math rendering */
.formula-display, .katex-display, .mjx-container {
    max-width: 100%;
    overflow-x: auto;
    overflow-y: hidden;
    -webkit-overflow-scrolling: touch;
}
.mjx-container svg {
    min-width: max-content;
    height: auto;
}
.math, .katex, .mjx-container {
    overflow-wrap: break-word;
    word-break: break-word;
}
/* Drawing Canvas */
#drawCanvas {
    position: fixed;
    top: 0;
    left: 0;
    z-index: 9998;
    touch-action: none;
    display: none;
    background: transparent; /* Trong suốt để thấy nội dung đề */
    width: 100vw;
    height: 100vh;
}

.toolbar {
    position: fixed;
    top: 100px;
    left: 10px;
    background: white;
    border-radius: 10px;
    box-shadow: 0 4px 12px rgba(0,0,0,0.2);
    padding: 6px;
    z-index: 10000; /* Cao hơn canvas */
    display: none;
    flex-direction: column;
    gap: 4px;
    width: 48px;
    align-items: center;
    transition: opacity 0.3s, transform 0.3s;
}

.toolbar.show {
    display: flex;
    opacity: 1;
    transform: translateY(0);
}

.toolbar button,
.toolbar select,
.toolbar input[type="color"],
.toolbar input[type="range"] {
    width: 100%;
    font-size: 10px;
    padding: 2px;
    height: 28px;
    box-sizing: border-box;
    background: var(--light-bg);
    border: 1px solid var(--primary-color);
    border-radius: 4px;
    cursor: pointer;
    transition: all 0.3s;
}

.toolbar button:hover,
.toolbar select:hover {
    background: var(--primary-color);
    color: white;
}

.floating-button {
    position: fixed;
    bottom: 20px;
    right: 20px;
    z-index: 10001; /* Cao hơn canvas và toolbar */
    background: var(--primary-color);
    color: white;
    border: none;
    padding: 12px 20px;
    border-radius: 50px;
    font-size: 16px;
    cursor: pointer;
    box-shadow: 0 4px 8px rgba(0,0,0,0.2);
    transition: all 0.3s;
}

.floating-button:hover {
    background: #a50e26;
    transform: translateY(-2px);
}

/* Responsive */
@media (max-width: 768px) {
    .toolbar {
        width: 40px;
        top: 80px;
        left: 5px;
    }
    .floating-button {
        bottom: 10px;
        right: 10px;
        padding: 8px 16px;
        font-size: 14px;
    }
}


/* Blackboard Toggle (góc trái dưới) */
.blackboard-toggle {
    position: fixed;
    bottom: 20px;
    left: 10px;
    z-index: 10020; /* Cao hơn header (1000), toolbar (10000), canvas (9998) */
    background: #c0153c !important; /* Đỏ để debug hiển thị */
    color: white !important;
    border: none;
    width: 32px;
    height: 32px;
    font-size: 18px;
    line-height: 32px;
    text-align: center;
    border-radius: 50%;
    cursor: pointer;
    box-shadow: 0 2px 4px rgba(0,0,0,0.2);
    display: block !important; /* Đảm bảo hiển thị */
}

.blackboard-toggle:hover, .blackboard-toggle.active {
    background: #a50e26 !important;
}

/* Blackboard */
.blackboard {
    position: fixed;
    top: 50%;
    left: 50%;
    transform: translate(-50%, -50%);
    width: 120vw;
    height: 60vh;
    max-width: 1200px;
    max-height: 600px;
    background: #004d00; /* Xanh đen bảng phấn */
    border: 5px solid #8B4513;
    border-radius: 10px;
    z-index: 9997; /* Thấp hơn canvas (9998) */
    display: none !important;
    cursor: move;
}

.blackboard.show {
    display: block !important;
}

/* Nút đóng bảng */
#close-blackboard {
    position: absolute;
    top: 10px;
    right: 10px;
    background: #c0153c;
    color: white;
    border: none;
    padding: 5px 10px;
    border-radius: 50%;
    cursor: pointer;
    z-index: 10001;
}

#close-blackboard:hover {
    background: #a50e26;
}

/* Responsive */
@media (max-width: 768px) {
    .blackboard {
        width: 90vw;
        height: 50vh;
        max-width: 400px;
        max-height: 300px;
    }
    .blackboard-toggle {
        width: 28px;
        height: 28px;
        font-size: 16px;
        line-height: 28px;
        bottom: 10px;
        left: 5px;
    }
}




/* Khi ở fullscreen: phóng toàn bộ nội dung */
/* Khi bật fullscreen thì mọi chữ đều to hơn */
.fullscreen-mode {
  font-size: 1.4em;   /* tăng toàn bộ chữ lên 1.4 lần */
  line-height: 1.6;
}

.fullscreen-mode h1 {
  font-size: 2.2em;
}
.fullscreen-mode h2 {
  font-size: 1.8em;
}