:root {
    --primary-gradient: linear-gradient(135deg, #667eea 0%, #764ba2 100%);
    --secondary-gradient: linear-gradient(45deg, #667eea, #764ba2);
    --accent-color: #667eea;
    --success-color: #4caf50;
    --text-primary: #333;
    --text-secondary: #666;
    --bg-white: #ffffff;
    --bg-light: #f8f9fa;
    --shadow-light: 0 10px 25px rgba(0,0,0,0.1);
    --shadow-heavy: 0 25px 50px rgba(0,0,0,0.15);
    --border-radius: 15px;
    --transition: all 0.3s ease;
}

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

body {
    font-family: 'Noto Sans KR', -apple-system, BlinkMacSystemFont, 'Segoe UI', sans-serif;
    line-height: 1.6;
    color: var(--text-primary);
    background: var(--primary-gradient);
    min-height: 100vh;
    overflow-x: hidden;
}

/* 헤더 스타일 */
.header {
    text-align: center;
    padding: 60px 20px 80px;
    background: rgba(255, 255, 255, 0.1);
    backdrop-filter: blur(20px);
    border-bottom: 1px solid rgba(255, 255, 255, 0.2);
    position: relative;
    z-index: 10;
}

/* 모바일 메뉴 버튼 */
.mobile-menu-btn {
    display: none;
    position: fixed;
    top: 20px;
    left: 20px;
    z-index: 1001;
    background: rgba(255, 255, 255, 0.95);
    border: none;
    border-radius: 12px;
    width: 50px;
    height: 50px;
    flex-direction: column;
    justify-content: center;
    align-items: center;
    gap: 5px;
    cursor: pointer;
    box-shadow: 0 4px 20px rgba(0, 0, 0, 0.15);
    backdrop-filter: blur(10px);
    transition: all 0.3s ease;
}

.mobile-menu-btn:hover {
    background: rgba(255, 255, 255, 1);
    transform: scale(1.05);
}

.mobile-menu-btn span {
    display: block;
    width: 22px;
    height: 3px;
    background: var(--accent-color);
    border-radius: 2px;
    transition: all 0.3s cubic-bezier(0.68, -0.55, 0.265, 1.55);
}

.mobile-menu-btn.active span:nth-child(1) {
    transform: rotate(45deg) translate(6px, 6px);
}

.mobile-menu-btn.active span:nth-child(2) {
    opacity: 0;
    transform: scale(0);
}

.mobile-menu-btn.active span:nth-child(3) {
    transform: rotate(-45deg) translate(6px, -6px);
}

/* 모바일 오버레이 */
.mobile-overlay {
    display: none;
    position: fixed;
    top: 0;
    left: 0;
    width: 100%;
    height: 100%;
    background: rgba(0, 0, 0, 0.6);
    z-index: 999;
    opacity: 0;
    transition: opacity 0.3s ease;
    backdrop-filter: blur(5px);
}

.mobile-overlay.active {
    opacity: 1;
}

.header h1 {
    font-size: clamp(2rem, 5vw, 3.5rem);
    color: white;
    margin-bottom: 20px;
    text-shadow: 0 4px 20px rgba(0, 0, 0, 0.3);
    font-weight: 700;
    letter-spacing: -0.02em;
}

.header p {
    font-size: clamp(1rem, 2.5vw, 1.3rem);
    color: rgba(255, 255, 255, 0.9);
    max-width: 700px;
    margin: 0 auto;
    font-weight: 400;
}

/* 메인 컨테이너 */
.main-container {
    display: flex;
    max-width: 1400px;
    margin: 0 auto;
    position: relative;
}

/* 사이드바 */
.sidebar {
    position: fixed;
    left: 0;
    top: 0;
    width: 350px;
    height: 100vh;
    background: rgba(255, 255, 255, 0.95);
    backdrop-filter: blur(20px);
    border-right: 1px solid rgba(0, 0, 0, 0.05);
    padding: 200px 30px 30px;
    box-shadow: 2px 0 30px rgba(0, 0, 0, 0.1);
    z-index: 100;
    overflow-y: auto;
}

.progress-container {
    position: relative;
    padding-left: 40px;
}

.progress-line {
    position: absolute;
    left: 18px;
    top: 0;
    width: 4px;
    height: 100%;
    background: linear-gradient(to bottom, #e0e0e0, #f5f5f5);
    border-radius: 2px;
}

.progress-fill {
    position: absolute;
    left: 18px;
    top: 0;
    width: 4px;
    background: var(--secondary-gradient);
    border-radius: 2px;
    transition: height 0.6s cubic-bezier(0.4, 0, 0.2, 1);
    height: 0%;
    box-shadow: 0 0 10px rgba(102, 126, 234, 0.3);
}

.nav-item {
    position: relative;
    margin-bottom: 35px;
    padding: 15px 20px;
    cursor: pointer;
    transition: var(--transition);
    border-radius: var(--border-radius);
    background: transparent;
}

.nav-item::before {
    content: '';
    position: absolute;
    left: -22px;
    top: 50%;
    transform: translateY(-50%);
    width: 20px;
    height: 20px;
    border-radius: 50%;
    background: #e0e0e0;
    border: 4px solid white;
    box-shadow: var(--shadow-light);
    transition: var(--transition);
    z-index: 2;
}

.nav-item.active {
    background: rgba(102, 126, 234, 0.1);
    transform: translateX(8px);
}

.nav-item.active::before {
    background: var(--secondary-gradient);
    transform: translateY(-50%) scale(1.2);
    box-shadow: 0 0 20px rgba(102, 126, 234, 0.4);
}

.nav-item.completed::before {
    background: var(--success-color);
    box-shadow: 0 0 15px rgba(76, 175, 80, 0.3);
}

.nav-title {
    font-weight: 600;
    font-size: 1.1rem;
    color: var(--text-primary);
    margin-bottom: 5px;
    letter-spacing: -0.01em;
}

.nav-subtitle {
    font-size: 0.9rem;
    color: var(--text-secondary);
    line-height: 1.4;
    font-weight: 400;
}

.nav-item.active .nav-title {
    color: var(--accent-color);
    font-weight: 700;
}

/* 메인 콘텐츠 */
.content {
    margin-left: 370px;
    padding: 40px;
    width: calc(100% - 370px);
    min-height: 100vh;
}

.study-section {
    margin-bottom: 120px;
    opacity: 0.7;
    transform: translateY(30px);
    transition: all 0.8s cubic-bezier(0.4, 0, 0.2, 1);
}

.study-section.active {
    opacity: 1;
    transform: translateY(0);
}

.study-section:first-child {
    opacity: 1;
    transform: translateY(0);
}

.section-card {
    background: var(--bg-white);
    border-radius: 25px;
    padding: 50px;
    box-shadow: var(--shadow-light);
    border: 1px solid rgba(255, 255, 255, 0.8);
    position: relative;
    overflow: hidden;
    transition: var(--transition);
}

.section-card::before {
    content: '';
    position: absolute;
    top: 0;
    left: 0;
    width: 100%;
    height: 6px;
    background: var(--secondary-gradient);
}

.section-card:hover {
    transform: translateY(-8px);
    box-shadow: var(--shadow-heavy);
}

.section-header {
    margin-bottom: 40px;
}

.section-number {
    display: inline-block;
    background: var(--secondary-gradient);
    color: white;
    padding: 10px 25px;
    border-radius: 30px;
    font-size: 0.9rem;
    font-weight: 600;
    margin-bottom: 20px;
    letter-spacing: 0.5px;
    text-transform: uppercase;
}

.section-title {
    font-size: clamp(1.8rem, 4vw, 2.5rem);
    color: var(--text-primary);
    margin-bottom: 15px;
    font-weight: 700;
    letter-spacing: -0.02em;
    line-height: 1.2;
}

.section-subtitle {
    font-size: 1.2rem;
    color: var(--text-secondary);
    margin-bottom: 30px;
    font-weight: 400;
    line-height: 1.5;
}

.duration-badge {
    display: inline-block;
    background: rgba(102, 126, 234, 0.1);
    color: var(--accent-color);
    padding: 8px 20px;
    border-radius: 25px;
    font-size: 0.9rem;
    font-weight: 600;
    margin-left: 15px;
    border: 1px solid rgba(102, 126, 234, 0.2);
}

.section-content {
    display: grid;
    grid-template-columns: repeat(auto-fit, minmax(300px, 1fr));
    gap: 25px;
    margin-bottom: 40px;
}

.content-block {
    background: var(--bg-light);
    padding: 30px;
    border-radius: var(--border-radius);
    border-left: 4px solid var(--accent-color);
    transition: var(--transition);
    position: relative;
}

.content-block:hover {
    transform: translateY(-5px);
    box-shadow: var(--shadow-light);
    border-left-color: #764ba2;
}

.content-block h4 {
    color: var(--text-primary);
    margin-bottom: 20px;
    font-size: 1.3rem;
    font-weight: 600;
    display: flex;
    align-items: center;
    gap: 10px;
}

.content-block p {
    color: var(--text-secondary);
    line-height: 1.7;
    margin-bottom: 15px;
}

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

.content-block li {
    padding: 10px 0;
    padding-left: 25px;
    position: relative;
    color: var(--text-secondary);
    line-height: 1.6;
    transition: var(--transition);
}

.content-block li::before {
    content: '▶';
    position: absolute;
    left: 0;
    color: var(--accent-color);
    font-size: 0.8rem;
    top: 50%;
    transform: translateY(-50%);
}

.content-block li:hover {
    color: var(--text-primary);
    padding-left: 30px;
}

.study-tips {
    background: var(--primary-gradient);
    color: white;
    padding: 35px;
    border-radius: var(--border-radius);
    margin-top: 30px;
    position: relative;
    overflow: hidden;
}

.study-tips h4 {
    margin-bottom: 15px;
    font-size: 1.3rem;
    font-weight: 600;
    display: flex;
    align-items: center;
    gap: 10px;
}

.study-tips p {
    opacity: 0.95;
    line-height: 1.7;
    font-weight: 400;
}

/* 반응형 디자인 */
@media (max-width: 1200px) {
    .sidebar {
        width: 300px;
        padding: 180px 25px 25px;
    }

    .content {
        margin-left: 320px;
        width: calc(100% - 320px);
    }
}

@media (max-width: 768px) {
    .mobile-menu-btn {
        display: flex !important;
    }

    .mobile-overlay {
        display: block !important;
    }

    .sidebar {
        transform: translateX(-100%);
        transition: transform 0.3s ease;
        width: 280px;
        z-index: 1000;
        border-right: none;
        box-shadow: none;
    }

    .sidebar.mobile-open {
        transform: translateX(0);
        box-shadow: 2px 0 30px rgba(0, 0, 0, 0.3);
    }

    .content {
        margin-left: 0;
        width: 100%;
        padding: 20px;
    }

    .section-content {
        grid-template-columns: 1fr;
        gap: 20px;
    }

    .section-card {
        padding: 30px 20px;
    }

    .header {
        padding: 80px 20px 60px;
    }

    .header h1 {
        margin-top: 20px;
    }
}

@media (max-width: 480px) {
    .sidebar {
        width: 260px;
        padding: 160px 20px 20px;
    }

    .section-card {
        padding: 25px 15px;
    }

    .content {
        padding: 15px;
    }

    .mobile-menu-btn {
        width: 45px;
        height: 45px;
        top: 15px;
        left: 15px;
    }

    .nav-item {
        margin-bottom: 25px;
        padding: 12px 15px;
    }

    .progress-container {
        padding-left: 35px;
    }
}

/* 스크롤바 스타일링 */
.sidebar::-webkit-scrollbar {
    width: 6px;
}

.sidebar::-webkit-scrollbar-track {
    background: transparent;
}

.sidebar::-webkit-scrollbar-thumb {
    background: rgba(102, 126, 234, 0.3);
    border-radius: 3px;
}

.sidebar::-webkit-scrollbar-thumb:hover {
    background: rgba(102, 126, 234, 0.5);
}

/* 접근성 개선 */
.nav-item:focus {
    outline: 2px solid var(--accent-color);
    outline-offset: 2px;
}

.content-block:focus-within {
    outline: 2px solid var(--accent-color);
    outline-offset: 2px;
}

/* 로딩 애니메이션 */
.loading {
    opacity: 0;
}

.loaded {
    opacity: 1;
}