:root {
            --main-bg: #ffffff;
            --accent-blue: #e0f2fe;
            --primary-blue: #0ea5e9;
            --accent-pink: #fdf2f8;
            --primary-pink: #ec4899;
            --text-dark: #334155;
            --text-light: #64748b;
            --border-color: #f1f5f9;
            --shadow: 0 4px 6px -1px rgb(0 0 0 / 0.1);
        }

        * {
            margin: 0;
            padding: 0;
            box-sizing: border-box;
            font-family: "Helvetica Neue", Arial, "Hiragino Kaku Gothic ProN", "Hiragino Sans", Meiryo, sans-serif;
        }

        body {
            background-color: var(--main-bg);
            color: var(--text-dark);
            line-height: 1.6;
        }

        /* Header */
        header {
            background-color: white;
            border-bottom: 2px solid var(--accent-blue);
            padding: 1rem 5%;
            display: flex;
            justify-content: space-between;
            align-items: center;
            position: sticky;
            top: 0;
            z-index: 100;
        }

        .btn-back {
            text-decoration: none;
            color: var(--text-light);
            font-size: 0.9rem;
            display: flex;
            align-items: center;
            gap: 5px;
        }

        .btn-back:hover { color: var(--primary-blue); }

        /* Course Banner */
        .course-banner {
            background: linear-gradient(135deg, var(--accent-blue) 0%, #f0f9ff 100%);
            padding: 4rem 5%;
            text-align: center;
        }

        .course-banner h1 {
            font-size: 2.5rem;
            margin-bottom: 0.5rem;
        }

        /* Main Grid Layout */
        main {
            max-width: 1100px;
            margin: 0 auto;
            padding: 3rem 5%;
            display: grid;
            grid-template-columns: 2fr 1fr;
            gap: 2rem;
        }

        .section-title {
            font-size: 1.25rem;
            margin-bottom: 1.5rem;
            display: flex;
            align-items: center;
            gap: 10px;
            padding-bottom: 0.5rem;
            border-bottom: 2px solid var(--accent-blue);
        }

        /* Lecture List */
        .lecture-list {
            display: flex;
            flex-direction: column;
            gap: 1rem;
        }

        .lecture-item {
            display: flex;
            align-items: center;
            justify-content: space-between;
            padding: 1.2rem;
            background: white;
            border: 1px solid var(--border-color);
            border-radius: 12px;
            text-decoration: none;
            color: inherit;
            transition: all 0.2s;
        }

        .lecture-item:hover {
            border-color: var(--primary-blue);
            box-shadow: var(--shadow);
            transform: translateX(5px);
        }

        .lecture-number {
            background: var(--accent-blue);
            color: var(--primary-blue);
            width: 40px;
            height: 40px;
            display: flex;
            align-items: center;
            justify-content: center;
            border-radius: 50%;
            font-weight: bold;
            font-size: 0.9rem;
            flex-shrink: 0;
        }

        .lecture-body {
            flex-grow: 1;
            margin: 0 1.5rem;
        }

        .lecture-body h3 {
            font-size: 1rem;
            margin-bottom: 2px;
        }

        .lecture-date {
            font-size: 0.8rem;
            color: var(--text-light);
        }

        .status-badge {
            font-size: 0.75rem;
            padding: 2px 8px;
            border-radius: 4px;
            background: #f1f5f9;
        }

        /* Sidebar Widgets */
        .sidebar {
            display: flex;
            flex-direction: column;
            gap: 2rem;
        }

        .widget {
            background: white;
            border: 1px solid var(--border-color);
            border-radius: 15px;
            padding: 1.5rem;
            box-shadow: 0 2px 4px rgba(0,0,0,0.02);
        }

        .widget h4 {
            font-size: 0.95rem;
            margin-bottom: 1rem;
            color: var(--text-dark);
            display: flex;
            align-items: center;
            gap: 8px;
        }

        .schedule-item {
            margin-bottom: 1rem;
            padding-left: 10px;
            border-left: 3px solid var(--primary-pink);
        }

        .schedule-date {
            font-size: 0.75rem;
            color: var(--primary-pink);
            font-weight: bold;
        }

        .schedule-title {
            font-size: 0.85rem;
            font-weight: 500;
        }


        /* Footer */
        footer {
            margin-top: 5rem;
            padding: 3rem 5%;
            text-align: center;
            background: #f8fafc;
            color: var(--text-light);
            font-size: 0.85rem;
        }

        @media (max-width: 850px) {
            main {
                grid-template-columns: 1fr;
            }
        }