:root {
    --color-primary: #667eea; /* 主色调，用于通用按钮等 */
    --color-secondary: #764ba2; /* 辅助色调 */
    --color-accent: #48c6ef; /* 强调色 */
    --text-dark: #2d3748; /* 深色文本 */
    --text-light: #f7fafc; /* 浅色文本 */
    --bg-light: rgba(255, 255, 255, 0.95); /* 浅色背景，带透明度 */
    --shadow-base: 0 20px 40px rgba(0, 0, 0, 0.15); /* 基础阴影 */
    --shadow-hover: 0 30px 60px rgba(0, 0, 0, 0.2); /* 悬停阴影 */
    --border-light: 1px solid rgba(255, 255, 255, 0.3); /* 浅色边框 */
    --grade1-hue: 330; /* 粉红色系 */
    --grade2-hue: 210; /* 蓝色系 */
    --grade3-hue: 39;  /* 橙黄色系 */
    --grade4-hue: 125; /* 绿色系 */
    --grade5-hue: 279; /* 紫色系 */
    --grade6-hue: 190; /* 青色系 */
}

/* Base and Global Styles */
* {
    margin: 0;
    padding: 0;
    box-sizing: border-box;
}

html, body {
    height: 100%;
    scroll-behavior: smooth;
}

body {
    font-family: 'Noto Sans SC', 'SF Pro Display', Arial, sans-serif;
    background: linear-gradient(135deg, 
        hsla(var(--grade1-hue), 80%, 90%, 0.2),
        hsla(var(--grade2-hue), 80%, 90%, 0.2) 20%,
        hsla(var(--grade3-hue), 80%, 90%, 0.2) 40%,
        hsla(var(--grade4-hue), 80%, 90%, 0.2) 60%,
        hsla(var(--grade5-hue), 80%, 90%, 0.2) 80%,
        hsla(var(--grade6-hue), 80%, 90%, 0.2) 100%
    );
    background-attachment: fixed;
    color: var(--text-dark);
    line-height: 1.6;
    -webkit-font-smoothing: antialiased;
}

/* Layout */
.container {
    max-width: 1400px;
    margin: 0 auto;
    padding: 2rem;
    position: relative;
    isolation: isolate;
}

/* Background Particles */
body::before {
    content: '';
    position: fixed;
    top: 0;
    left: 0;
    width: 100%;
    height: 100%;
    background: url("data:image/svg+xml,%3Csvg width='60' height='60' viewBox='0 0 60 60' xmlns='http://www.w3.org/2000/svg'%3E%3Cg fill='none' fill-rule='evenodd'%3E%3Cg fill='%23ffffff' fill-opacity='0.08'%3E%3Ccircle cx='30' cy='30' r='1'/%3E%3C/g%3E%3C/g%3E%3C/svg%3E") repeat;
    z-index: -1;
    animation: float 20s infinite linear;
}

@keyframes float {
    0% { transform: translate(0, 0); }
    50% { transform: translate(-5px, -10px); }
    100% { transform: translate(0, 0); }
}

/* Header */
.app-header {
    text-align: center;
    margin-bottom: 4rem;
    padding: 3rem;
    background: var(--bg-light);
    border-radius: 30px;
    box-shadow: var(--shadow-base), inset 0 1px 0 rgba(255, 255, 255, 0.6);
    position: relative;
    backdrop-filter: blur(20px);
    border: var(--border-light);
    overflow: hidden;
    transition: all 0.5s cubic-bezier(0.2, 0.8, 0.2, 1);
}

.app-header::before {
    content: '';
    position: absolute;
    top: 0;
    left: -100%;
    width: 100%;
    height: 100%;
    background: linear-gradient(90deg, transparent, rgba(255, 255, 255, 0.3), transparent);
    animation: shimmer 3s infinite;
}

@keyframes shimmer {
    0% { left: -100%; }
    100% { left: 100%; }
}

.header-text {
    position: relative;
    z-index: 2;
}

.app-header h1 {
    font-size: clamp(2rem, 5vw, 3.5rem);
    margin-bottom: 0.5rem;
    font-weight: 700;
    background: linear-gradient(135deg, var(--text-dark), #1a202c);
    -webkit-background-clip: text;
    -webkit-text-fill-color: transparent;
    text-shadow: none;
}

.app-header p {
    font-size: clamp(1rem, 2vw, 1.3em);
    font-weight: 400;
    opacity: 0.8;
}

/* 返回按钮 */
.back-btn {
    position: absolute;
    left: 0.8rem;
    top: 75%;
    transform: translateY(-50%);
    background: linear-gradient(135deg, var(--color-primary), var(--color-secondary));
    color: white;
    border: none;
    padding: 0.75rem 1.5rem;
    border-radius: 50px;
    cursor: pointer;
    font-size: 1rem;
    font-weight: 500;
    transition: all 0.4s cubic-bezier(0.4, 0, 0.2, 1);
    box-shadow: 0 10px 30px rgba(102, 126, 234, 0.3);
    display: flex;
    align-items: center;
    gap: 0.5rem;
    opacity: 0;
    pointer-events: none;
    z-index: 3;
    -webkit-appearance: none;
    -moz-appearance: none;
    appearance: none;
}

.back-btn.active {
    opacity: 1;
    pointer-events: auto;
}

.back-btn.grade-1 { background: linear-gradient(135deg, hsl(var(--grade1-hue), 80%, 70%), hsl(var(--grade1-hue), 80%, 55%)); }
.back-btn.grade-2 { background: linear-gradient(135deg, hsl(var(--grade2-hue), 80%, 70%), hsl(var(--grade2-hue), 80%, 55%)); }
.back-btn.grade-3 { background: linear-gradient(135deg, hsl(var(--grade3-hue), 80%, 70%), hsl(var(--grade3-hue), 80%, 55%)); }
.back-btn.grade-4 { background: linear-gradient(135deg, hsl(var(--grade4-hue), 80%, 70%), hsl(var(--grade4-hue), 80%, 55%)); }
.back-btn.grade-5 { background: linear-gradient(135deg, hsl(var(--grade5-hue), 80%, 70%), hsl(var(--grade5-hue), 80%, 55%)); }
.back-btn.grade-6 { background: linear-gradient(135deg, hsl(var(--grade6-hue), 80%, 70%), hsl(var(--grade6-hue), 80%, 55%)); }

.back-btn:hover {
    transform: translateY(-50%) translateX(-0.5rem) scale(1.05);
    box-shadow: 0 15px 40px rgba(102, 126, 234, 0.4);
}

.back-btn svg {
    width: 1.2rem;
    height: 1.2rem;
}

/* Grids */
.homepage-grid {
    display: grid;
    grid-template-columns: repeat(auto-fit, minmax(350px, 1fr));
    gap: 2.5rem;
    justify-content: center;
    transition: all 0.5s ease-in-out;
}

.homepage-grid:not(.active) {
    display: none;
}

/* Cards */
.grade-card {
    position: fixed;
    border-radius: 2rem;
    padding: 3.5rem 2.5rem;
    text-align: center;
    cursor: pointer;
    transition: all 0.5s cubic-bezier(0.2, 0.8, 0.2, 1);
    box-shadow: var(--shadow-base), inset 0 1px 0 rgba(255, 255, 255, 0.2);
    color: white;
    position: relative;
    overflow: hidden;
    backdrop-filter: blur(20px);
}

.grade-card:hover {
    transform: translateY(-1rem) scale(1.03);
    box-shadow: var(--shadow-hover), inset 0 1px 0 rgba(255, 255, 255, 0.3);
}

.grade-card::before {
    content: '';
    position: absolute;
    top: -50%;
    left: -50%;
    width: 200%;
    height: 200%;
    background: linear-gradient(45deg, transparent, rgba(255, 255, 255, 0.1), transparent);
    transform: rotate(45deg);
    transition: transform 0.8s cubic-bezier(0.4, 0, 0.2, 1);
    opacity: 0;
}

.grade-card:hover::before {
    transform: rotate(45deg) translate(100%, 100%);
    opacity: 1;
}

.grade-number {
    font-size: 5rem;
    font-weight: 800;
    line-height: 1;
    margin-bottom: 1.25rem;
    text-shadow: 0 5px 15px rgba(0, 0, 0, 0.3);
}

.grade-title {
    font-size: 2rem;
    margin-bottom: 0.5rem;
    font-weight: 700;
}

.grade-desc {
    font-size: 1rem;
    opacity: 0.9;
    font-weight: 400;
}

/* 年级卡片颜色 */
.grade-card:nth-child(1) { background: linear-gradient(135deg, hsl(var(--grade1-hue), 80%, 70%), hsl(var(--grade1-hue), 80%, 55%)); }
.grade-card:nth-child(2) { background: linear-gradient(135deg, hsl(var(--grade2-hue), 80%, 70%), hsl(var(--grade2-hue), 80%, 55%)); }
.grade-card:nth-child(3) { background: linear-gradient(135deg, hsl(var(--grade3-hue), 80%, 70%), hsl(var(--grade3-hue), 80%, 55%)); }
.grade-card:nth-child(4) { background: linear-gradient(135deg, hsl(var(--grade4-hue), 80%, 70%), hsl(var(--grade4-hue), 80%, 55%)); }
.grade-card:nth-child(5) { background: linear-gradient(135deg, hsl(var(--grade5-hue), 80%, 70%), hsl(var(--grade5-hue), 80%, 55%)); }
.grade-card:nth-child(6) { background: linear-gradient(135deg, hsl(var(--grade6-hue), 80%, 70%), hsl(var(--grade6-hue), 80%, 55%)); }

/* Grade Content */
.grade-content {
    display: none;
    background: var(--bg-light);
    border-radius: 2rem;
    padding: 2.5rem;
    box-shadow: var(--shadow-base), inset 0 1px 0 rgba(255, 255, 255, 0.6);
    backdrop-filter: blur(20px);
    border: var(--border-light);
    animation: fadeInUp 0.6s cubic-bezier(0.4, 0, 0.2, 1);
}

.grade-content.active {
    display: block;
}

@keyframes fadeInUp {
    from { opacity: 0; transform: translateY(2rem); }
    to { opacity: 1; transform: translateY(0); }
}

.content-title {
    font-size: clamp(1.5rem, 3vw, 2.5rem);
    margin-bottom: 2.5rem;
    text-align: center;
    padding-bottom: 1.25rem;
    border-bottom: 3px solid;
    border-image: linear-gradient(135deg, var(--color-primary), var(--color-secondary), var(--color-accent)) 1;
    font-weight: 700;
    color: var(--text-dark);
}

/* 单元标题 */
.unit-title {
    font-size: 1.2rem;
    margin: 1.5rem 0;
    padding: 1.5rem 2rem;
    border-radius: 1.5rem;
    cursor: pointer;
    transition: all 0.4s cubic-bezier(0.2, 0.8, 0.2, 1);
    font-weight: 500;
    position: relative;
    overflow: hidden;
    box-shadow: 0 8px 25px rgba(0, 0, 0, 0.08), inset 0 1px 0 rgba(255, 255, 255, 0.8);
    backdrop-filter: blur(20px);
    border: 1px solid rgba(255, 255, 255, 0.6);
    display: flex;
    align-items: center;
    color: var(--text-dark);
}

.grade-content#grade-1 .unit-title { background: hsla(var(--grade1-hue), 80%, 95%, 0.7); }
.grade-content#grade-2 .unit-title { background: hsla(var(--grade2-hue), 80%, 95%, 0.7); }
.grade-content#grade-3 .unit-title { background: hsla(var(--grade3-hue), 80%, 95%, 0.7); }
.grade-content#grade-4 .unit-title { background: hsla(var(--grade4-hue), 80%, 95%, 0.7); }
.grade-content#grade-5 .unit-title { background: hsla(var(--grade5-hue), 80%, 95%, 0.7); }
.grade-content#grade-6 .unit-title { background: hsla(var(--grade6-hue), 80%, 95%, 0.7); }

.unit-title::before {
    content: '';
    position: absolute;
    top: 0;
    left: 0;
    width: 6px;
    height: 100%;
    background: linear-gradient(135deg, var(--color-primary), var(--color-secondary));
    transition: width 0.5s cubic-bezier(0.4, 0, 0.2, 1);
}

.grade-content#grade-1 .unit-title::before { background: linear-gradient(135deg, hsl(var(--grade1-hue), 80%, 70%), hsl(var(--grade1-hue), 80%, 55%)); }
.grade-content#grade-2 .unit-title::before { background: linear-gradient(135deg, hsl(var(--grade2-hue), 80%, 70%), hsl(var(--grade2-hue), 80%, 55%)); }
.grade-content#grade-3 .unit-title::before { background: linear-gradient(135deg, hsl(var(--grade3-hue), 80%, 70%), hsl(var(--grade3-hue), 80%, 55%)); }
.grade-content#grade-4 .unit-title::before { background: linear-gradient(135deg, hsl(var(--grade4-hue), 80%, 70%), hsl(var(--grade4-hue), 80%, 55%)); }
.grade-content#grade-5 .unit-title::before { background: linear-gradient(135deg, hsl(var(--grade5-hue), 80%, 70%), hsl(var(--grade5-hue), 80%, 55%)); }
.grade-content#grade-6 .unit-title::before { background: linear-gradient(135deg, hsl(var(--grade6-hue), 80%, 70%), hsl(var(--grade6-hue), 80%, 55%)); }

.unit-title::after {
    content: '▼';
    position: absolute;
    right: 1.5rem;
    font-size: 0.8em;
    color: rgba(var(--color-primary), 0.7);
    transition: transform 0.3s ease;
    top: 50%;
    transform: translateY(-50%);
}

.unit-title:hover {
    transform: translateY(-5px);
    box-shadow: 0 15px 40px rgba(102, 126, 234, 0.1), inset 0 1px 0 rgba(255, 255, 255, 0.9);
}

.unit-title.active {
    background: hsla(var(--grade-active-hue, 220), 80%, 90%, 0.8);
    color: var(--text-dark);
    box-shadow: 0 12px 30px rgba(102, 126, 234, 0.2), inset 0 1px 0 rgba(255, 255, 255, 0.6);
}

.grade-content#grade-1 .unit-title.active { background: hsla(var(--grade1-hue), 80%, 90%, 0.8); }
.grade-content#grade-2 .unit-title.active { background: hsla(var(--grade2-hue), 80%, 90%, 0.8); }
.grade-content#grade-3 .unit-title.active { background: hsla(var(--grade3-hue), 80%, 90%, 0.8); }
.grade-content#grade-4 .unit-title.active { background: hsla(var(--grade4-hue), 80%, 90%, 0.8); }
.grade-content#grade-5 .unit-title.active { background: hsla(var(--grade5-hue), 80%, 90%, 0.8); }
.grade-content#grade-6 .unit-title.active { background: hsla(var(--grade6-hue), 80%, 90%, 0.8); }

.unit-title.active::after {
    transform: translateY(-50%) rotate(180deg);
}

.unit-icon {
    display: inline-flex;
    justify-content: center;
    align-items: center;
    width: 35px;
    height: 35px;
    background: linear-gradient(135deg, var(--color-primary), var(--color-accent));
    border-radius: 50%;
    margin-right: 1rem;
    color: white;
    font-weight: bold;
    box-shadow: 0 4px 15px rgba(102, 126, 234, 0.3);
    flex-shrink: 0;
}

.grade-content#grade-1 .unit-icon { background: linear-gradient(135deg, hsl(var(--grade1-hue), 80%, 70%), hsl(var(--grade1-hue), 80%, 55%)); }
.grade-content#grade-2 .unit-icon { background: linear-gradient(135deg, hsl(var(--grade2-hue), 80%, 70%), hsl(var(--grade2-hue), 80%, 55%)); }
.grade-content#grade-3 .unit-icon { background: linear-gradient(135deg, hsl(var(--grade3-hue), 80%, 70%), hsl(var(--grade3-hue), 80%, 55%)); }
.grade-content#grade-4 .unit-icon { background: linear-gradient(135deg, hsl(var(--grade4-hue), 80%, 70%), hsl(var(--grade4-hue), 80%, 55%)); }
.grade-content#grade-5 .unit-icon { background: linear-gradient(135deg, hsl(var(--grade5-hue), 80%, 70%), hsl(var(--grade5-hue), 80%, 55%)); }
.grade-content#grade-6 .unit-icon { background: linear-gradient(135deg, hsl(var(--grade6-hue), 80%, 70%), hsl(var(--grade6-hue), 80%, 55%)); }

/* Video Grid */
.video-grid {
    display: none;
    grid-template-columns: repeat(auto-fit, minmax(300px, 1fr));
    gap: 2rem;
    margin-top: 2.5rem;
}

.video-grid.active {
    display: grid;
    animation: fadeInUp 0.6s cubic-bezier(0.4, 0, 0.2, 1);
}

/* 视频卡片 */
.video-card {
    border-radius: 1.5rem;
    padding: 1.5rem;
    box-shadow: 0 15px 35px rgba(0, 0, 0, 0.1), inset 0 1px 0 rgba(255, 255, 255, 0.6);
    transition: all 0.4s cubic-bezier(0.4, 0, 0.2, 1);
    backdrop-filter: blur(20px);
    border: var(--border-light);
    position: relative;
    overflow: hidden;
    color: var(--text-dark);
}

.video-card:hover {
    transform: translateY(-0.5rem) scale(1.02);
    box-shadow: 0 25px 50px rgba(0, 0, 0, 0.15), inset 0 1px 0 rgba(255, 255, 255, 0.8);
}

.video-card::before {
    content: '';
    position: absolute;
    top: 0;
    left: -100%;
    width: 100%;
    height: 100%;
    background: linear-gradient(90deg, transparent, rgba(255, 255, 255, 0.1), transparent);
    transition: left 0.8s;
}

.video-card:hover::before {
    left: 100%;
}

.video-title {
    font-size: 1.25rem;
    margin-bottom: 1.25rem;
    text-align: center;
    font-weight: 600;
    position: relative;
    z-index: 2;
    text-shadow: none;
}

.grade-content#grade-1 .video-card { background: hsla(var(--grade1-hue), 80%, 95%, 0.6); }
.grade-content#grade-2 .video-card { background: hsla(var(--grade2-hue), 80%, 95%, 0.6); }
.grade-content#grade-3 .video-card { background: hsla(var(--grade3-hue), 80%, 95%, 0.6); }
.grade-content#grade-4 .video-card { background: hsla(var(--grade4-hue), 80%, 95%, 0.6); }
.grade-content#grade-5 .video-card { background: hsla(var(--grade5-hue), 80%, 95%, 0.6); }
.grade-content#grade-6 .video-card { background: hsla(var(--grade6-hue), 80%, 95%, 0.6); }

.video-wrapper {
    position: relative;
    width: 100%;
    padding-top: 56.25%;
    border-radius: 1rem;
    overflow: hidden;
    background: rgba(0, 0, 0, 0.2);
}

.video-wrapper video {
    position: absolute;
    top: 0;
    left: 0;
    width: 100%;
    height: 100%;
    border-radius: 1rem;
    outline: none;
    z-index: 2;
    background: #000;
}

.video-placeholder {
    position: absolute;
    top: 0;
    left: 0;
    width: 100%;
    height: 100%;
    background: rgba(0, 0, 0, 0.3);
    display: flex;
    align-items: center;
    justify-content: center;
    border-radius: 1rem;
    z-index: 3;
    cursor: pointer;
    transition: opacity 0.3s ease;
}

.video-placeholder.hidden {
    opacity: 0;
    pointer-events: none;
}

.play-icon {
    width: 3rem;
    height: 3rem;
    background: rgba(255, 255, 255, 0.95);
    border-radius: 50%;
    display: flex;
    align-items: center;
    justify-content: center;
    font-size: 2rem;
    color: var(--color-primary);
    box-shadow: 0 8px 30px rgba(0, 0, 0, 0.2);
    transition: transform 0.3s ease;
    padding-left: 3px;
}

.pause-icon {
    position: absolute;
    bottom: 10px;
    left: 10px;
    width: 2rem;
    height: 2rem;
    background: rgba(255, 255, 255, 0.95);
    border-radius: 50%;
    display: flex;
    align-items: center;
    justify-content: center;
    font-size: 1rem;
    color: var(--color-primary);
    box-shadow: 0 8px 30px rgba(0, 0, 0, 0.2);
    transition: transform 0.3s ease;
    cursor: pointer;
    z-index: 4;
}

.pause-icon.hidden {
    opacity: 0;
    pointer-events: none;
}

.grade-content#grade-1 .play-icon, .grade-content#grade-1 .pause-icon { color: hsl(var(--grade1-hue), 80%, 55%); }
.grade-content#grade-2 .play-icon, .grade-content#grade-2 .pause-icon { color: hsl(var(--grade2-hue), 80%, 55%); }
.grade-content#grade-3 .play-icon, .grade-content#grade-3 .pause-icon { color: hsl(var(--grade3-hue), 80%, 55%); }
.grade-content#grade-4 .play-icon, .grade-content#grade-4 .pause-icon { color: hsl(var(--grade4-hue), 80%, 55%); }
.grade-content#grade-5 .play-icon, .grade-content#grade-5 .pause-icon { color: hsl(var(--grade5-hue), 80%, 55%); }
.grade-content#grade-6 .play-icon, .grade-content#grade-6 .pause-icon { color: hsl(var(--grade6-hue), 80%, 55%); }

.video-placeholder:hover .play-icon {
    transform: scale(1.1);
}

.pause-icon:hover {
    transform: scale(1.1);
}

/* Coming Soon */
.coming-soon {
    text-align: center;
    color: var(--text-dark);
    font-size: clamp(1.4rem, 2.5vw, 1.8em);
    padding: 5rem 2rem;
    background: linear-gradient(135deg, rgba(248, 249, 250, 0.9), rgba(233, 236, 239, 0.9));
    border-radius: 25px;
    border: 2px dashed rgba(102, 126, 234, 0.3);
    backdrop-filter: blur(20px);
    font-weight: 600;
}

.coming-soon::before {
    content: "🎬";
    display: block;
    font-size: 5em;
    margin-bottom: 1.5rem;
    animation: bounce 2s infinite;
}

@keyframes bounce {
    0%, 20%, 50%, 80%, 100% { transform: translateY(0); }
    40% { transform: translateY(-20px); }
    60% { transform: translateY(-10px); }
}

/* Responsive Design */
@media (max-width: 1200px) {
    .homepage-grid, .video-grid {
        gap: 1.5rem;
    }
}

@media (max-width: 2000px) {
    .container {
        padding: 1rem;
    }
    
    .app-header {
        padding: 1.5rem;
        border-radius: 1.5rem;
        margin-bottom: 2rem;
    }

    .back-btn {
        left: 1rem;
        padding: 0.6rem 1rem;
        font-size: 0.9rem;
    }

    .homepage-grid {
        grid-template-columns: minmax(350px, 1fr);
        justify-content: center;
        gap: 1.5rem;
        overflow-x: auto;
        padding: 0 1rem;
    }

    .grade-card {
        padding: 3.5rem 2.5rem;
        border-radius: 2rem;
        min-width: 350px;
    }
    
    .grade-number {
        font-size: 5rem;
    }

    .unit-title {
        padding: 1rem 1.5rem;
        font-size: 1rem;
        margin: 1rem 0;
        border-radius: 1rem;
    }
    
    .unit-icon {
        width: 30px;
        height: 30px;
        margin-right: 0.75rem;
    }

    .video-grid {
        grid-template-columns: 1fr;
        gap: 1.5rem;
        margin-top: 1.5rem;
    }

    .video-card {
        padding: 1rem;
        border-radius: 1rem;
    }

    .play-icon {
        width: 2rem;
        height: 2rem;
        font-size: 1.5rem;
    }

    .pause-icon {
        width: 1.5rem;
        height: 1.5rem;
        font-size: 0.8rem;
    }
}

@media (max-width: 480px) {
    .homepage-grid {
        grid-template-columns: minmax(350px, 350px);
        justify-content: center;
        overflow-x: auto;
        padding: 0 1rem;
    }

    .grade-card {
        min-width: 350px;
        margin: 0 auto;
    }
}