/* CSS 变量 */
:root {
    --primary-color: #262627;
    --secondary-color: #6e7072;
    --background-color: #f4f4f9;
    --text-color: #333;
    --white: #fff;
    --shadow: 0 4px 6px rgba(0, 0, 0, 0.1);
    --radius: 10px;
    --transition: all 0.3s ease;
}

/* 全局样式 */
* {
    margin: 0;
    padding: 0;
    box-sizing: border-box;
    font-family: 'Segoe UI', Tahoma, Geneva, Verdana, sans-serif;
}

body {
    background-color: var(--background-color);
    color: var(--text-color);
}

/* 导航栏 */
.nav {
    height: 60px;
    background-color: var(--primary-color);
    box-shadow: var(--shadow);
    position: fixed;
    top: 0;
    left: 0;
    right: 0;
    z-index: 1000;
    display: flex;
    justify-content: center;
    align-items: center;
}

.ullist {
    display: flex;
    list-style: none;
}

.libt {
    margin: 0 15px;
    position: relative;
}

.libt a {
    color: var(--white);
    text-decoration: none;
    padding: 10px 20px;
    display: block;
    transition: var(--transition);
    border-radius: var(--radius);
}

.libt a:hover {
    background-color: var(--secondary-color);
}

.droplist {
    background-color: var(--secondary-color);
    box-shadow: var(--shadow);
    display: none;
    position: absolute;
    top: 100%;
    left: 0;
    width: 200px;
    border-radius: var(--radius);
    overflow: hidden;
    opacity: 0;
    transform: translateY(-10px);
    transition: var(--transition);
}

.libt:hover .droplist {
    display: block;
    opacity: 1;
    transform: translateY(0);
}

.droplist li {
    border-top: 1px solid var(--primary-color);
}

.droplist li a {
    padding: 10px;
    display: block;
    color: var(--white);
    transition: var(--transition);
}

.droplist li a:hover {
    background-color: var(--primary-color);
}

/* 搜索框 */
.search-container {
    position: absolute;
    right: 20px;
    top: 50%;
    transform: translateY(-50%);
    display: flex;
    align-items: center;
}

.search-box {
    width: 200px;
    padding: 10px;
    border: none;
    border-radius: 25px;
    outline: none;
    background-color: rgba(255, 255, 255, 0.2);
    color: var(--white);
    transition: var(--transition);
}

.search-box:focus {
    width: 250px;
    background-color: rgba(255, 255, 255, 0.3);
}

.search-icon {
    width: 20px;
    height: 20px;
    margin-left: 10px;
    cursor: pointer;
}

/* 页面内容 */
.content {
    padding-top: 100px;
    text-align: center;
}

h2 {
    font-size: 40px;
    color: var(--primary-color);
    margin-bottom: 20px;
}

/* 题库卡片 */
.card-container {
    display: grid;
    grid-template-columns: repeat(auto-fit, minmax(300px, 1fr));
    gap: 20px;
    padding: 20px;
    max-width: 1200px;
    margin: auto;
}

.card {
    display: block;
    text-decoration: none;  /* 去除默认下划线 */
    color: inherit;  /* 继承父元素文字颜色 */
    background-color: var(--white);
    border-radius: var(--radius);
    box-shadow: var(--shadow);
    padding: 20px;
    transition: var(--transition);
}

.card:hover {
    transform: translateY(-5px);
    box-shadow: 0 8px 12px rgba(0, 0, 0, 0.2);
}

.card h3 {
    font-size: 24px;
    color: var(--primary-color);
    margin-bottom: 10px;
}

.card p {
    font-size: 14px;
    color: var(--text-color);
}

/* 加载动画 */
@keyframes spin {
    0% { transform: rotate(0deg); }
    100% { transform: rotate(360deg); }
}

.loader {
    border: 4px solid #f3f3f3;
    border-top: 4px solid var(--primary-color);
    border-radius: 50%;
    width: 40px;
    height: 40px;
    animation: spin 1s linear infinite;
    margin: 20px auto;
}