:root {
    --blue-5: #2eabff;
    --blue-6: #09f;
    --blue-7: #0082d9;
    --background-white: #fff;
    --text-dark: #222;
}

/* 全局样式 */
* {
    margin: 0;
    padding: 0;
    box-sizing: border-box;
}

body {
    font-family: -apple-system, BlinkMacSystemFont, 'Segoe UI', Roboto, 'Helvetica Neue', Arial;
    color: var(--text-dark);
    background: #000; /* 背景色保证过渡 */
    overflow-x: hidden;
    font-size: 1.5rem;   /* 默认正文从 16px 提升到大约 19px */
    line-height: 1.8; 
}

/* 导航栏 */
header {
    position: fixed;
    top: 0;
    left: 0;
    width: 100%;
    z-index: 1000;
    transition: background 0.3s ease, box-shadow 0.3s ease;
}

/* 默认透明渐变 */
header.transparent {
    background: linear-gradient(to bottom, rgba(0,0,0,0.6), rgba(0,0,0,0));
    backdrop-filter: blur(6px);
}

/* 滚动后背景加深，带阴影 */
header.scrolled {
    background: rgba(0, 0, 0, 0.9);
    backdrop-filter: blur(8px);
    box-shadow: 0 2px 12px rgba(0, 0, 0, 0.6);
}

nav {
    display: flex;
    justify-content: center; /* 居中导航 */
    align-items: center;
    height: 70px;
}

/* 导航链接样式 */
.nav-links {
    list-style: none;
    display: flex;
    gap: 3rem;
    margin: 0;
    padding: 0;
}

.nav-links a {
    color: white;
    font-size: 1.6rem;
    font-weight: 600;
    text-decoration: none;
    position: relative;
    padding: 8px 0;
    transition: color 0.3s ease;
    letter-spacing: 0.5px;
}

/* 下划线动画 */
.nav-links a::after {
    content: "";
    position: absolute;
    left: 50%;
    bottom: -6px;
    width: 0;
    height: 2px;
    background: var(--blue-5);
    transition: all 0.3s ease;
    transform: translateX(-50%);
}

.nav-links a:hover {
    color: var(--blue-5);
}

.nav-links a:hover::after {
    width: 100%;
}

/* 当前 section 高亮 */
.nav-links a.active {
    color: var(--blue-5);
}

.nav-links a.active::after {
    width: 100%;
}

/* 每个 section 占满全屏 */
main section {
    width: 100vw;
    height: 100vh;
    display: flex;
    flex-direction: column;
    justify-content: center;
    align-items: center;
    text-align: center;
    
    background: rgba(255,255,255,0.85);
    border-radius: 12px;
    padding: 20px;
    margin: 0; /* 全屏不留 margin */
    
    opacity: 0;
    transition: opacity 0.8s ease, transform 0.8s ease;
}

/* 欢迎全屏盒子特殊背景 */
.welcome {
    background: linear-gradient(rgba(0,0,0,0.4), rgba(0,0,0,0.2)),
                url("https://picsum.photos/1600/900?blur=3") no-repeat center center fixed;
    background-size: cover;
    color: white;
    height: 100vh;
    width: 100vw;
    display: flex;
    flex-direction: column;
    justify-content: center;
    align-items: center;
    text-align: center;
    
}

.welcome h1 {
    font-size: 6rem;
    text-shadow: 0 6px 15px rgba(0,0,0,0.7);
}

/* 向下箭头 */
.scroll-down {
    margin-top: 20px;
    font-size: 2rem;
    color: white;
    animation: bounce 1.5s infinite;
    cursor: pointer;
}

@keyframes bounce {
    0%,100% { transform: translateY(0); }
    50% { transform: translateY(10px); }
}

/* 动画效果 */
.fade-in { opacity: 1; transform: translateY(0); }
.about-section {
    width: 100vw;
    height: 100vh;
    display: flex;
    flex-direction: column;
    justify-content: center;
    align-items: center;
    text-align: center;
    background: rgba(255, 255, 255, 0.9);
    padding: 40px;
    line-height: 1.8;
    font-size: 1.2rem;
}

.about-section h1 {
    font-size: 3rem;
    margin-bottom: 2rem;
    color: var(--blue-7);
}