/* ===== 全局样式 ===== */
body {
    font-family: 'Inter', -apple-system, BlinkMacSystemFont, 'Segoe UI', Roboto, 'Microsoft YaHei', sans-serif;
    background-color: #f5f9fe;
    color: #1e2f4b;
    line-height: 1.5;
}

/* 全局容器 */
.container {
    max-width: 1200px;
    margin: 0 auto;
    padding: 2rem 1.5rem;
}

/* ===== 头部样式 ===== */
.header {
    text-align: center;
    border-bottom: 2px solid rgba(0, 107, 164, 0.15);
    margin-bottom: 3rem;
}

.header h1 {
    font-size: 2.8rem;
    font-weight: 600;
    background: linear-gradient(145deg, #1e3c5f, #0f2b40);
    -webkit-background-clip: text;
    -webkit-text-fill-color: transparent;
    background-clip: text;
    margin-bottom: 0.75rem;
    letter-spacing: -0.02em;
}

.header .desc {
    font-size: 1.2rem;
    color: #5271a0;
    font-weight: 400;
    display: inline-block;
    padding-bottom: 0.6rem;
    border-bottom: 2px solid transparent;
}

/* ===== 课列表 ===== */
.lesson-list {
    display: grid;
    grid-template-columns: repeat(auto-fill, minmax(350px, 1fr));
    gap: 2rem;
    margin-bottom: 4rem;
}

/* 课卡片 */
.lesson-item {
    background: #ffffff;
    border-radius: 1.5rem;
    padding: 2rem 1.8rem;
    box-shadow: 0 15px 30px -12px rgba(0, 55, 100, 0.12);
    transition: all 0.3s cubic-bezier(0.2, 0, 0, 1);
    border: 1px solid rgba(255,255,255,0.5);
    backdrop-filter: blur(2px);
    display: flex;
    flex-direction: column;
}

.lesson-item:hover {
    transform: translateY(-8px);
    box-shadow: 0 25px 40px -14px rgba(0, 80, 130, 0.25);
    border-color: rgba(0, 107, 164, 0.2);
}

.lesson-item h2 {
    font-size: 1.9rem;
    font-weight: 550;
    color: #0a3b5c;
    margin-bottom: 1rem;
    padding-bottom: 0.75rem;
    border-bottom: 3px solid rgba(0, 107, 164, 0.15);
    line-height: 1.2;
}

.lesson-item .intro {
    font-size: 1rem;
    color: #3f546d;
    line-height: 1.7;
    margin-bottom: 2rem;
    flex: 1;
}

/* ===== 按钮组 ===== */
.btn-group {
    display: flex;
    gap: 1rem;
    margin-top: auto;
}

.btn {
    display: inline-block;
    padding: 0.8rem 1.2rem;
    border-radius: 3rem;
    font-size: 1rem;
    font-weight: 530;
    text-align: center;
    border: 1.5px solid transparent;
    box-shadow: 0 4px 10px rgba(0, 60, 100, 0.08);
    transition: all 0.2s ease;
    flex: 1 1 0;
    text-decoration: none;
    color: #fff;
}

/* 主按钮（查看知识点） */
.btn:first-child {
    background: #006ba4;
    border-color: #006ba4;
}

.btn:first-child:hover {
    background: #004b7c;
    border-color: #004b7c;
    transform: scale(1.02);
    box-shadow: 0 12px 20px -10px #006ba4;
}

/* 演示按钮 */
.demo-btn {
    background: #e6f2fa;
    color: #006ba4;
    border-color: #b8d1e6;
}

.demo-btn:hover {
    background: #d0e6f5;
    border-color: #006ba4;
    transform: scale(1.02);
    box-shadow: 0 12px 20px -10px rgba(0,107,164,0.25);
}

/* ===== 页脚 ===== */
.footer {
    text-align: center;
    padding: 2rem 0 1rem;
    border-top: 2px solid rgba(0, 80, 120, 0.1);
    color: #5f7895;
    font-size: 0.95rem;
    font-weight: 380;
    letter-spacing: 0.3px;
}

/* ===== 响应式适配 ===== */
@media (max-width: 768px) {
    .lesson-list {
        grid-template-columns: 1fr;
    }

    .btn-group {
        flex-direction: column;
    }

    .lesson-item {
        padding: 1.8rem 1.5rem;
    }

    .lesson-item h2 {
        font-size: 1.7rem;
    }
}

@media (max-width: 480px) {
    .container {
        padding: 1.5rem 1rem;
    }
}