/* =========================================
   MathOS V3.0 - 独立介绍页样式 (intro.css)
   ========================================= */

/* 引入字体：权威衬线体 + 清晰无衬线体 + 图标库 */
@import url('https://fonts.googleapis.com/css2?family=Merriweather:ital,wght@0,300;0,400;0,700;0,900;1,300&family=Inter:wght@300;400;500;600&family=Material+Symbols+Outlined:opsz,wght,FILL,GRAD@24,300,0,0&display=swap');

/* 核心变量 */
:root {
    --royal-blue: #002147;     /* 牛津蓝 */
    --gold-accent: #C5A059;    /* 鎏金 */
    --paper-white: #FCFBF9;    /* 米白 */
    --text-dark: #1A1A1A;      /* 深灰 */
    --text-grey: #555555;      /* 浅灰 */
    
    --font-heading: 'Merriweather', serif; 
    --font-body: 'Inter', sans-serif;
}

/* 基础重置 */
body {
    background-color: var(--paper-white);
    color: var(--text-dark);
    font-family: var(--font-body);
    line-height: 1.8;
    margin: 0;
    overflow-x: hidden;
    -webkit-font-smoothing: antialiased;
}

/* 顶部导航 */
nav.floating-nav {
    position: absolute;
    top: 0;
    left: 0;
    width: 100%;
    padding: 32px 48px;
    z-index: 100;
    display: flex;
    justify-content: space-between;
    align-items: center;
    box-sizing: border-box;
}

.brand-white {
    font-family: var(--font-heading);
    font-weight: 900;
    color: white; 
    font-size: 1.2rem;
    letter-spacing: 1px;
    opacity: 0.9;
}

/* 头部区域 */
.header-section {
    position: relative;
    width: 100%;
    background-color: var(--royal-blue);
    padding: 140px 24px 100px 24px;
    box-sizing: border-box;
    overflow: hidden;
    display: flex;
    justify-content: center;
}

#particle-canvas {
    position: absolute;
    top: 0;
    left: 0;
    width: 100%;
    height: 100%;
    z-index: 1; 
    pointer-events: none;
}

.header-content {
    position: relative;
    z-index: 2;
    max-width: 800px;
    text-align: left;
}

.header-title {
    font-family: var(--font-heading);
    font-size: 3.5rem; 
    color: #FFFFFF;
    margin: 0 0 32px 0;
    line-height: 1.1;
    font-weight: 300;
}

.header-summary {
    font-family: var(--font-heading);
    font-size: 1.15rem;
    color: rgba(255, 255, 255, 0.85);
    line-height: 1.8;
    font-weight: 300;
    font-style: italic;
    border-left: 2px solid var(--gold-accent);
    padding-left: 24px;
    margin: 0;
}

/* 内容区域 */
.main-container {
    max-width: 800px;
    margin: 0 auto;
    padding: 80px 24px 120px 24px;
    position: relative;
    z-index: 4;
    background-color: var(--paper-white);
}

/* 风琴式列表 */
.accordion-item {
    border-bottom: 1px solid rgba(0,0,0,0.1); 
    transition: background-color 0.3s ease;
}

.accordion-header {
    padding: 32px 0;
    cursor: pointer;
    display: flex;
    justify-content: space-between;
    align-items: center;
}

.accordion-title-group {
    display: flex;
    flex-direction: column;
    gap: 8px;
}

.accordion-index {
    font-size: 0.85rem;
    color: var(--gold-accent);
    font-weight: 700;
    text-transform: uppercase;
    letter-spacing: 0.1em;
}

.accordion-title {
    font-family: var(--font-heading);
    font-size: 1.8rem;
    color: var(--royal-blue);
    font-weight: 400;
    transition: color 0.3s ease;
}

.accordion-header:hover .accordion-title {
    color: var(--gold-accent);
}

.accordion-icon {
    font-size: 2rem;
    color: rgba(0,0,0,0.2);
    transition: transform 0.4s cubic-bezier(0.2, 0, 0, 1), color 0.3s;
    user-select: none;
}

.accordion-header:hover .accordion-icon {
    color: var(--royal-blue);
}

.accordion-content {
    max-height: 0;
    overflow: hidden;
    transition: max-height 0.6s cubic-bezier(0.2, 0, 0, 1), opacity 0.4s;
    opacity: 0;
}

.content-inner {
    padding-bottom: 48px;
    padding-top: 0;
    color: var(--text-grey);
    font-size: 1.05rem;
}

.content-inner p { margin-bottom: 24px; }
.content-inner ul { list-style: none; padding: 0; }
.content-inner li {
    position: relative;
    padding-left: 24px;
    margin-bottom: 12px;
}
.content-inner li::before {
    content: "•";
    color: var(--gold-accent);
    position: absolute;
    left: 0;
    font-weight: bold;
}

/* 激活状态 */
.accordion-item.active .accordion-icon {
    transform: rotate(45deg);
    color: var(--gold-accent);
}
.accordion-item.active .accordion-title {
    color: var(--royal-blue);
    font-weight: 700;
}
.accordion-item.active .accordion-content {
    opacity: 1;
}