/* ===== Sunny学数学 基础样式 ===== */
:root {
    --primary: #667eea;
    --primary-dark: #5a6fd6;
    --secondary: #764ba2;
    --bg-gradient: linear-gradient(135deg, #667eea 0%, #764ba2 100%);
    --card-bg: rgba(255,255,255,0.12);
    --card-bg-hover: rgba(255,255,255,0.18);
    --text: #fff;
    --text-muted: rgba(255,255,255,0.7);
    --border: rgba(255,255,255,0.2);
    --radius: 16px;
    --radius-sm: 10px;
}

* { margin: 0; padding: 0; box-sizing: border-box; }

body {
    font-family: -apple-system, BlinkMacSystemFont, "Segoe UI", Roboto, "Helvetica Neue", Arial, "Noto Sans SC", "PingFang SC", "Microsoft YaHei", sans-serif;
    background: var(--bg-gradient);
    min-height: 100vh;
    color: var(--text);
    display: flex;
    flex-direction: column;
    align-items: center;
}

/* ===== Navigation ===== */
nav {
    width: 100%;
    display: flex;
    justify-content: center;
    padding: 16px 24px;
    border-bottom: 1px solid var(--border);
}
.nav-inner {
    max-width: 800px;
    width: 100%;
    display: flex;
    justify-content: space-between;
    align-items: center;
}
.nav-brand {
    font-size: 20px;
    font-weight: 700;
    color: var(--text);
    text-decoration: none;
    display: flex;
    align-items: center;
    gap: 8px;
}
.nav-links {
    display: flex;
    gap: 20px;
    list-style: none;
}
.nav-links a {
    color: var(--text);
    text-decoration: none;
    font-size: 14px;
    opacity: 0.85;
    padding: 4px 8px;
    border-radius: 6px;
    transition: background 0.2s, opacity 0.2s;
}
.nav-links a:hover {
    background: var(--card-bg);
    opacity: 1;
}
.nav-links a.active {
    opacity: 1;
    border-bottom: 2px solid #fff;
}

/* ===== Container ===== */
.container {
    max-width: 800px;
    width: 100%;
    padding: 40px 24px;
    flex: 1;
}

/* ===== Header ===== */
header {
    text-align: center;
    margin-bottom: 40px;
}
.logo { font-size: 64px; margin-bottom: 12px; }
h1 {
    font-size: 38px;
    font-weight: 700;
    margin-bottom: 8px;
    letter-spacing: 2px;
}
.subtitle { font-size: 16px; opacity: 0.9; line-height: 1.6; }
.breadcrumb {
    font-size: 13px;
    opacity: 0.65;
    margin-bottom: 24px;
}
.breadcrumb a {
    color: var(--text);
    opacity: 0.7;
    text-decoration: none;
}
.breadcrumb a:hover { opacity: 1; text-decoration: underline; }

/* ===== Article Cards (homepage) ===== */
.article-grid {
    display: grid;
    grid-template-columns: 1fr;
    gap: 20px;
    margin-bottom: 40px;
}
.article-card {
    background: var(--card-bg);
    backdrop-filter: blur(10px);
    border-radius: var(--radius);
    padding: 28px 24px;
    text-decoration: none;
    color: var(--text);
    display: block;
    transition: transform 0.2s, background 0.2s;
    border: 1px solid transparent;
}
.article-card:hover {
    transform: translateY(-3px);
    background: var(--card-bg-hover);
    border-color: var(--border);
}
.article-card .tag {
    display: inline-block;
    font-size: 12px;
    padding: 2px 10px;
    border-radius: 12px;
    background: rgba(255,255,255,0.2);
    margin-bottom: 10px;
}
.article-card h2 { font-size: 22px; margin-bottom: 8px; }
.article-card p { font-size: 14px; opacity: 0.8; line-height: 1.6; margin-bottom: 12px; }
.article-card .read-more {
    font-size: 13px;
    opacity: 0.7;
}
.article-card .read-more:hover { opacity: 1; }

/* ===== Article Content Page ===== */
.article-content {
    background: var(--card-bg);
    backdrop-filter: blur(10px);
    border-radius: var(--radius);
    padding: 40px 32px;
    margin-bottom: 40px;
    border: 1px solid var(--border);
}
.article-content h1 {
    font-size: 32px;
    margin-bottom: 8px;
    text-align: left;
}
.article-content .meta {
    font-size: 13px;
    opacity: 0.6;
    margin-bottom: 28px;
}
.article-content h2 {
    font-size: 24px;
    margin: 32px 0 16px;
    padding-bottom: 8px;
    border-bottom: 1px solid var(--border);
}
.article-content h3 {
    font-size: 19px;
    margin: 24px 0 12px;
}
.article-content p {
    font-size: 16px;
    line-height: 1.8;
    margin-bottom: 16px;
    opacity: 0.92;
}
.article-content ul, .article-content ol {
    margin: 12px 0 20px 24px;
    line-height: 1.8;
    font-size: 16px;
    opacity: 0.92;
}
.article-content li { margin-bottom: 8px; }
.article-content .formula {
    text-align: center;
    font-size: 20px;
    font-weight: 600;
    padding: 20px;
    margin: 20px 0;
    background: rgba(255,255,255,0.08);
    border-radius: var(--radius-sm);
    font-family: "Times New Roman", "STIX", serif;
    letter-spacing: 1px;
}
.article-content .example-box {
    background: rgba(255,255,255,0.06);
    border-left: 4px solid rgba(255,255,255,0.5);
    border-radius: 0 var(--radius-sm) var(--radius-sm) 0;
    padding: 20px 24px;
    margin: 20px 0;
}
.article-content .example-box .label {
    font-size: 14px;
    font-weight: 600;
    opacity: 0.8;
    margin-bottom: 8px;
}
.article-content .example-box p { margin-bottom: 8px; }
.article-content strong { opacity: 1; }
.article-content .tip-box {
    background: rgba(255, 215, 0, 0.12);
    border-radius: var(--radius-sm);
    padding: 16px 20px;
    margin: 16px 0;
    font-size: 14px;
    border: 1px solid rgba(255, 215, 0, 0.25);
}

/* ===== Feature Cards (homepage) ===== */
.features {
    display: grid;
    grid-template-columns: repeat(auto-fit, minmax(220px, 1fr));
    gap: 20px;
    margin-bottom: 40px;
}
.feature-card {
    background: var(--card-bg);
    backdrop-filter: blur(10px);
    border-radius: var(--radius);
    padding: 28px 20px;
    text-align: center;
    transition: transform 0.2s, background 0.2s;
    text-decoration: none;
    color: var(--text);
    display: block;
}
.feature-card:hover {
    transform: translateY(-4px);
    background: var(--card-bg-hover);
}
.feature-icon { font-size: 40px; margin-bottom: 12px; }
.feature-title { font-size: 18px; font-weight: 600; margin-bottom: 8px; }
.feature-desc { font-size: 13px; opacity: 0.8; line-height: 1.6; }

/* ===== Math Quote ===== */
.math-quote {
    text-align: center;
    font-size: 18px;
    font-style: italic;
    opacity: 0.8;
    margin-bottom: 40px;
    padding: 20px;
    border-left: 3px solid rgba(255,255,255,0.4);
}
.math-quote cite {
    display: block;
    margin-top: 6px;
    font-size: 13px;
    opacity: 0.65;
    font-style: normal;
}

/* ===== Footer ===== */
footer {
    text-align: center;
    padding: 24px;
    font-size: 13px;
    opacity: 0.5;
    width: 100%;
}

/* ===== Quiz Interactive ===== */
.quiz-item {
    margin: 20px 0;
}
.quiz-item p { margin-bottom: 10px; }
.quiz-row {
    display: flex;
    align-items: center;
    gap: 10px;
    flex-wrap: wrap;
    margin: 8px 0;
}
.quiz-input {
    background: rgba(255,255,255,0.12);
    border: 1px solid var(--border);
    border-radius: 8px;
    padding: 8px 14px;
    color: #fff;
    font-size: 15px;
    font-family: inherit;
    width: 140px;
    outline: none;
    transition: border-color 0.2s;
}
.quiz-input:focus { border-color: rgba(255,255,255,0.6); }
.quiz-input::placeholder { color: rgba(255,255,255,0.35); }
.quiz-input:disabled { opacity: 0.5; }
.quiz-btn {
    background: rgba(255,255,255,0.18);
    border: 1px solid var(--border);
    border-radius: 8px;
    padding: 8px 18px;
    color: #fff;
    font-size: 14px;
    cursor: pointer;
    transition: background 0.2s;
    font-family: inherit;
}
.quiz-btn:hover { background: rgba(255,255,255,0.28); }
.quiz-btn:disabled { cursor: default; }
.quiz-feedback {
    font-size: 14px;
    margin-top: 6px;
    min-height: 24px;
}
.quiz-hint {
    display: none;
    background: rgba(255, 215, 0, 0.1);
    border: 1px solid rgba(255, 215, 0, 0.2);
    border-radius: 8px;
    padding: 12px 16px;
    margin-top: 10px;
    font-size: 14px;
    line-height: 1.6;
}

/* ===== Daily Banner ===== */
.daily-banner {
    background: linear-gradient(135deg, rgba(255,215,0,0.15), rgba(255,165,0,0.1));
    border: 1px solid rgba(255,215,0,0.3);
    border-radius: var(--radius);
    padding: 24px;
    margin-bottom: 32px;
    text-align: center;
}
.daily-banner .label {
    font-size: 13px;
    opacity: 0.7;
    margin-bottom: 4px;
}
.daily-banner h2 { font-size: 22px; margin-bottom: 8px; }
.daily-banner p { font-size: 14px; opacity: 0.85; line-height: 1.6; }
.daily-banner .btn-sm {
    display: inline-block;
    margin-top: 12px;
    background: rgba(255,255,255,0.2);
    color: #fff;
    padding: 8px 24px;
    border-radius: 20px;
    text-decoration: none;
    font-size: 14px;
    transition: background 0.2s;
}
.daily-banner .btn-sm:hover { background: rgba(255,255,255,0.3); }

/* ===== Back Link ===== */
.back-link {
    display: inline-block;
    margin-bottom: 20px;
    color: var(--text);
    opacity: 0.7;
    text-decoration: none;
    font-size: 14px;
}
.back-link:hover { opacity: 1; }

/* ===== Responsive ===== */
@media (max-width: 600px) {
    h1 { font-size: 28px; }
    .logo { font-size: 48px; }
    .container { padding: 24px 16px; }
    .nav-inner { flex-direction: column; gap: 12px; }
    .nav-links { gap: 12px; flex-wrap: wrap; justify-content: center; }
    .features { grid-template-columns: 1fr; }
    .article-content { padding: 24px 16px; }
    .article-content h1 { font-size: 26px; }
}