/* base config */
        * {
            margin: 0;
            padding: 0;
            box-sizing: border-box;
            font-family: 'Segoe UI', Tahoma, Geneva, Verdana, sans-serif;
        }

        body {
            line-height: 1.6;
            color: #333;
            background: linear-gradient(135deg, #f5f7fa 0%, #e4e8f0 100%);
            min-height: 100vh;
        }

        .container {
            width: 90%;
            max-width: 1200px;
            margin: 0 auto;
            padding: 0 20px;
        }

        /* nav bar */
        header {
            background-color: #fff;
            box-shadow: 0 2px 15px rgba(0, 0, 0, 0.1);
            position: sticky;
            top: 0;
            z-index: 1000;
            padding: 0.5rem 0;
        }

        nav {
            display: flex;
            justify-content: space-between;
            align-items: center;
            padding: 0.5rem 0;
        }

        /* school logo image */
        .logo {
            display: flex;
            align-items: center;
            text-decoration: none;
            transition: opacity 0.3s ease;
        }

        .logo:hover {
            opacity: 0.8;
        }

        .logo-img {
            height: 60px;
            width: auto;
        }

        /* nav links */
        .nav-links {
            display: flex;
            list-style: none;
            gap: 2.5rem;
            align-items: center;
        }

        .nav-links a {
            text-decoration: none;
            color: #333;
            font-weight: 600;
            font-size: 1.1rem;
            padding: 0.5rem 0;
            position: relative;
            transition: color 0.3s ease;
        }

        .nav-links a:hover,
        .nav-links a.active {
            color: #56b4a6;
        }

        /* nav bar active action*/
        .nav-links a.active::after {
            content: '';
            position: absolute;
            bottom: 0;
            left: 0;
            width: 100%;
            height: 3px;
            background-color: #56b4a6;
            border-radius: 2px;
        }

        /* Language Switcher */
        .language-switcher {
            display: flex;
            align-items: center;
            margin-left: 1rem;
        }

        .lang-btn {
            background: none;
            border: 2px solid #56b4a6;
            color: #56b4a6;
            padding: 0.4rem 0.8rem;
            border-radius: 20px;
            cursor: pointer;
            font-weight: 600;
            transition: all 0.3s ease;
            margin-left: 0.5rem;
        }

        .lang-btn.active {
            background-color: #56b4a6;
            color: white;
        }

        .lang-btn:hover {
            background-color: #6de1d0;
            border-color: #6de1d0;
            color: white;
        }

        /* footer [contacts] */
        footer {
            text-align: center;
            padding: 1.5rem 0;
            background-color: #333;
            color: #fff;
            margin-top: 2rem;
        }

        /* contents */
        main {
            padding: 3rem 0;
            min-height: calc(100vh - 160px);
        }

        /* Course Nav Bar */
        .course-nav-container {
            background: white;
            border-radius: 12px;
            padding: 1.5rem;
            margin-bottom: 2rem;
            box-shadow: 0 5px 15px rgba(0, 0, 0, 0.05);
        }

        .course-nav-title {
            text-align: center;
            font-size: 1.8rem;
            margin-bottom: 1.5rem;
            color: #6a4b90;
            font-weight: 600;
        }

        .course-nav {
            display: flex;
            justify-content: center;
            flex-wrap: wrap;
            gap: 1rem;
        }
        
        .course-nav-item {
            padding: 12px 25px;
            background: #f8f9fa;
            border-radius: 30px;
            cursor: pointer;
            font-weight: 600;
            transition: all 0.3s ease;
            border: 2px solid transparent;
            display: flex;
            align-items: center;
            gap: 8px;
        }
        
        .course-nav-item:hover, 
        .course-nav-item.active {
            background: #6a4b90;
            color: white;
            transform: translateY(-3px);
            box-shadow: 0 5px 15px rgba(0, 0, 0, 0.1);
        }
        
        /* contents */
        .content-section {
            margin-bottom: 3rem;
            padding: 2rem;
            background: white;
            border-radius: 12px;
            box-shadow: 0 5px 15px rgba(0, 0, 0, 0.05);
            transition: transform 0.3s ease;
        }

        .content-section:hover {
            transform: translateY(-5px);
        }
        
        .section-title {
            color: #6a4b90;
            font-size: 1.5rem;
            margin-bottom: 1.5rem;
            padding-bottom: 0.5rem;
            border-bottom: 2px solid #e8f5e9;
            display: flex;
            align-items: center;
            gap: 10px;
        }

        /* Course Intro section */               
        .course-details {
            max-height: none;
            overflow: visible;
        }
        
        .detail-item {
            margin-bottom: 2rem;
            padding: 1.5rem;
            background: white;
            border-radius: 8px;
            box-shadow: 0 3px 10px rgba(0, 0, 0, 0.05);
        }
        
        .detail-title {
            margin-bottom: 1rem;
            color: #56b4a6;
            font-size: 1.3rem;
            font-weight: 600;
            display: flex;
            align-items: center;
            gap: 10px;
        }
        
        .image-gallery {
            display: flex;
            flex-wrap: wrap;
            gap: 15px;
            margin: 1.5rem 0;
        }
        
        .gallery-item {
            width: calc(33.333% - 15px);
            border-radius: 8px;
            overflow: hidden;
            box-shadow: 0 3px 10px rgba(0, 0, 0, 0.1);
            transition: transform 0.3s ease;
        }

        .gallery-item:hover {
            transform: scale(1.03);
        }
        
        .gallery-item img {
            width: 100%;
            height: 180px;
            object-fit: cover;
            display: block;
        }
        
        .video-grid {
            display: grid;
            grid-template-columns: repeat(auto-fill, minmax(300px, 1fr));
            gap: 1.5rem;
            margin-bottom: 2rem;
        }
        
        .video-card {
            border: 1px solid #eaeaea;
            border-radius: 12px;
            overflow: hidden;
            cursor: pointer;
            transition: transform 0.3s ease, box-shadow 0.3s ease;
            background: white;
        }
        
        .video-card:hover {
            transform: translateY(-5px);
            box-shadow: 0 10px 25px rgba(0,0,0,0.1);
        }

        /* topup videos styling */
        .video-card.pinned {
            border: 3px solid #6a4b90;
            position: relative;
        }
        
        .video-card.pinned::before {
            content: "★";
            position: absolute;
            top: 10px;
            right: 10px;
            color: yellow;
            padding: 3px 8px;
            font-size: 1.5rem;
            font-weight: bold;
            z-index: 2;
        }
        
        .video-thumbnail {
            position: relative;
            height: 180px;
            overflow: hidden;
        }
        
        .video-thumbnail img {
            width: 100%;
            height: 100%;
            object-fit: cover;
            transition: transform 0.5s ease;
        }

        .video-card:hover .video-thumbnail img {
            transform: scale(1.1);
        }
        
        .play-icon {
            position: absolute;
            top: 50%;
            left: 50%;
            transform: translate(-50%, -50%);
            width: 60px;
            height: 60px;
            background: rgba(86, 180, 166, 0.9);
            border-radius: 50%;
            display: flex;
            align-items: center;
            justify-content: center;
            transition: all 0.3s ease;
        }

        .video-card:hover .play-icon {
            background: rgba(106, 75, 144, 0.9);
            transform: translate(-50%, -50%) scale(1.1);
        }
        
        .play-icon::after {
            content: '';
            border-style: solid;
            border-width: 12px 0 12px 20px;
            border-color: transparent transparent transparent #ffffff;
            margin-left: 5px;
        }
        
        .video-info {
            padding: 1.5rem;
        }
        
        .video-title {
            color: #56b4a6;
            margin-bottom: 0.5rem;
            font-size: 1.2rem;
            font-weight: 600;
        }
        
        .video-meta {
            display: flex;
            justify-content: space-between;
            font-size: 0.9rem;
            color: #666;
            margin-bottom: 0.8rem;
        }
        
        .video-description {
            font-size: 0.95rem;
            color: #555;
            line-height: 1.5;
        }
        
        .load-more {
            display: flex;
            justify-content: center;
            text-align: center;
            margin: 2rem 0;
        }
        
        .load-more-btn {
            background: #6a4b90;
            color: white;
            border: none;
            padding: 0.8rem 1.8rem;
            border-radius: 30px;
            cursor: pointer;
            font-size: 1rem;
            font-weight: 600;
            transition: all 0.3s ease;
            display: flex;
            align-items: center;
            gap: 8px;
        }

        .load-more-btn:hover {
            transform: translateY(-3px);
            box-shadow: 0 5px 15px rgba(96, 68, 130, 0.3);
        }
        
        .loading-indicator {
            text-align: center;
            padding: 1.5rem;
            display: none;
            color: #6a4b90;
            font-size: 1.1rem;
        }
        
        .loading-indicator.active {
            display: block;
        }

        .loading-indicator i {
            animation: spin 1s linear infinite;
            margin-right: 10px;
        }

        @keyframes spin {
            0% { transform: rotate(0deg); }
            100% { transform: rotate(360deg); }
        }
        
        .modal-overlay {
            position: fixed;
            top: 0;
            left: 0;
            width: 100%;
            height: 100%;
            background: rgba(0,0,0,0.7);
            display: flex;
            align-items: center;
            justify-content: center;
            z-index: 1000;
            opacity: 0;
            visibility: hidden;
            transition: all 0.3s;
        }
        
        .modal-overlay.active {
            opacity: 1;
            visibility: visible;
        }
        
        .modal-content {
            background: white;
            width: 90%;
            max-width: 800px;
            border-radius: 12px;
            overflow: hidden;
            max-height: 90vh;
            display: flex;
            flex-direction: column;
            box-shadow: 0 20px 40px rgba(0, 0, 0, 0.2);
        }
        
        .modal-header {
            display: flex;
            justify-content: space-between;
            align-items: center;
            padding: 1.5rem;
            border-bottom: 1px solid #eaeaea;
            background: #f9f9f9;
        }

        .modal-title {
            color: #6a4b90;
            font-size: 1.4rem;
            font-weight: 600;
        }
        
        .close-modal {
            background: none;
            border: none;
            font-size: 1.8rem;
            cursor: pointer;
            color: #666;
            transition: color 0.3s ease;
        }

        .close-modal:hover {
            color: #56b4a6;
        }
        
        .video-player-container {
            position: relative;
            padding-bottom: 56.25%; /* 16:9 aspect ratio */
            height: 0;
            overflow: hidden;
        }
        
        .bilibili-embed {
            position: absolute;
            top: 0;
            left: 0;
            width: 100%;
            height: 100%;
        }
        
        .bilibili-embed iframe {
            width: 100%;
            height: 100%;
            border: none;
        }
        
        .tutorial-content {
            padding: 1.5rem;
            max-height: 200px;
            overflow-y: auto;
            background: #f9f9f9;
        }

        /* Our People */
        .teacher-intro {
            font-size: 1rem;
            color: #56b4a6;
            margin-bottom: 0.5rem;
            font-weight: 600;
        }
        .teachers-gallery {
            display: grid;
            grid-template-columns: repeat(auto-fill, minmax(280px, 1fr));
            gap: 2rem;
            margin-top: 1.5rem;
        }
        
        .teacher-card {
            background: white;
            border-radius: 12px;
            overflow: hidden;
            box-shadow: 0 5px 15px rgba(0, 0, 0, 0.08);
            transition: transform 0.3s ease;
            position: relative;
        }
        
        .teacher-card:hover {
            transform: translateY(-5px);
        }
        
        .teacher-image {
            width: 100%;
            height: 250px;
            overflow: hidden;
        }
        
        .teacher-image img {
            width: 100%;
            height: 100%;
            object-fit: cover;
            transition: transform 0.5s ease;
        }
        
        .teacher-card:hover .teacher-image img {
            transform: scale(1.05);
        }
        
        .teacher-info {
            padding: 1.5rem;
            text-align: center;
        }
        
        .teacher-name {
            font-size: 1.3rem;
            color: #56b4a6;
            margin-bottom: 0.5rem;
            font-weight: 600;
        }
        
        .teacher-role {
            color: #666;
            font-style: italic;
            margin-bottom: 1rem;
            font-size: 1rem;
        }
        
        .teacher-bio {
            position: absolute;
            bottom: 0;
            left: 0;
            right: 0;
            background: rgba(106, 75, 144, 0.95);
            color: white;
            padding: 1.5rem;
            transform: translateY(100%);
            transition: transform 0.3s ease;
            max-height: 80%;
            overflow-y: auto;
            border-top-left-radius: 12px;
            border-top-right-radius: 12px;
        }
        
        .teacher-card:hover .teacher-bio {
            transform: translateY(0);
        }
        
        footer {
            background: #333;
            color: white;
            text-align: center;
            padding: 2rem 0;
            margin-top: 3rem;
        }

        footer p {
            margin: 0;
            font-size: 1rem;
        }
        
        /* reactive design for mobiles */
        @media (max-width: 768px) {
            nav {
                flex-direction: column;
                gap: 1rem;
            }
            
            .nav-links {
                flex-wrap: wrap;
                justify-content: center;
            }
            
            .page-header h1 {
                font-size: 2rem;
            }
            
            .page-header p {
                font-size: 1rem;
            }
            
            .course-card-header {
                flex-direction: column;
                align-items: flex-start;
                text-align: center;
            }
            
            .course-image {
                margin: 0 auto 1rem;
            }
            
            .gallery-item {
                width: calc(50% - 10px);
            }
            
            .video-grid {
                grid-template-columns: 1fr;
            }
            
            .teachers-gallery {
                grid-template-columns: 1fr;
            }

            .course-nav {
                flex-direction: column;
                align-items: center;
            }

            .course-nav-item {
                width: 100%;
                max-width: 300px;
                justify-content: center;
            }
        }

        @media (max-width: 480px) {
            .gallery-item {
                width: 100%;
            }
            
            .content-section {
                padding: 1.5rem;
            }
            
            .course-details-content {
                padding: 1.5rem;
            }
        }