/* ===== CSS Variables for Blue-Green Tech Theme ===== */
:root {
    /* Primary Colors - Blue-Green Spectrum */
    --primary-dark: #023E8A;        /* Deep Blue */
    --primary: #0077B6;             /* Medium Blue */
    --primary-light: #00B4D8;       /* Cyan-Blue */
    --accent: #2EC4B6;              /* Teal-Green */
    --accent-light: #90E0EF;        /* Light Cyan */

    /* Neutral Colors */
    --bg-white: #FFFFFF;
    --bg-light: #F8F9FA;
    --bg-gray: #E9ECEF;
    --text-dark: #212529;
    --text-medium: #495057;
    --text-light: #6C757D;
    --text-white: #FFFFFF;

    /* Semantic Colors */
    --gradient-1: linear-gradient(135deg, #023E8A 0%, #0077B6 50%, #2EC4B6 100%);
    --gradient-2: linear-gradient(135deg, #00B4D8 0%, #2EC4B6 100%);
    --shadow-sm: 0 2px 4px rgba(2, 62, 138, 0.08);
    --shadow-md: 0 4px 12px rgba(2, 62, 138, 0.12);
    --shadow-lg: 0 8px 24px rgba(2, 62, 138, 0.15);

    /* Typography */
    --font-primary: -apple-system, BlinkMacSystemFont, 'Segoe UI', 'Roboto', 'Helvetica Neue', Arial, sans-serif;
    --font-secondary: 'Georgia', 'Times New Roman', serif;

    /* Spacing */
    --section-padding: 100px 0;
    --container-width: 1200px;
}

/* ===== Reset & Base Styles ===== */
* {
    margin: 0;
    padding: 0;
    box-sizing: border-box;
}

html {
    scroll-behavior: smooth;
    scroll-padding-top: 80px;
}

body {
    font-family: var(--font-primary);
    color: var(--text-dark);
    line-height: 1.7;
    overflow-x: hidden;
    background-color: var(--bg-white);
}

.container {
    max-width: var(--container-width);
    margin: 0 auto;
    padding: 0 20px;
}

/* ===== Typography ===== */
h1, h2, h3, h4, h5, h6 {
    font-weight: 700;
    line-height: 1.3;
    margin-bottom: 1rem;
}

p {
    margin-bottom: 1rem;
}

.zh-text {
    color: var(--text-light);
    font-weight: 400;
}

/* ===== Navigation ===== */
.navbar {
    position: fixed;
    top: 0;
    left: 0;
    width: 100%;
    background: rgba(255, 255, 255, 0.98);
    backdrop-filter: blur(10px);
    box-shadow: var(--shadow-sm);
    z-index: 1000;
    transition: all 0.3s ease;
}

.navbar.scrolled {
    box-shadow: var(--shadow-md);
}

.nav-container {
    max-width: var(--container-width);
    margin: 0 auto;
    padding: 1rem 20px;
    display: flex;
    justify-content: space-between;
    align-items: center;
}

.nav-logo img {
    height: 50px;
    width: auto;
    transition: transform 0.3s ease;
}

.nav-logo img:hover {
    transform: scale(1.05);
}

.nav-menu {
    display: flex;
    list-style: none;
    gap: 2.5rem;
}

.nav-link {
    text-decoration: none;
    color: var(--text-dark);
    font-weight: 600;
    font-size: 0.95rem;
    position: relative;
    padding: 0.5rem 0;
    transition: color 0.3s ease;
}

.nav-link::after {
    content: '';
    position: absolute;
    bottom: 0;
    left: 0;
    width: 0;
    height: 2px;
    background: var(--gradient-2);
    transition: width 0.3s ease;
}

.nav-link:hover {
    color: var(--primary);
}

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

.hamburger {
    display: none;
    flex-direction: column;
    cursor: pointer;
    gap: 5px;
}

.hamburger span {
    width: 25px;
    height: 3px;
    background: var(--text-dark);
    border-radius: 3px;
    transition: all 0.3s ease;
}

/* ===== Hero Section ===== */
.hero {
    position: relative;
    height: 100vh;
    display: flex;
    align-items: center;
    justify-content: center;
    overflow: hidden;
    background: linear-gradient(135deg, #023E8A 0%, #0077B6 50%, #2EC4B6 100%);
}

.hero-background {
    position: absolute;
    top: 0;
    left: 0;
    width: 100%;
    height: 100%;
    z-index: 1;
}

#heroCanvas {
    width: 100%;
    height: 100%;
    display: block;
}

.hero-overlay {
    position: absolute;
    top: 0;
    left: 0;
    width: 100%;
    height: 100%;
    background: radial-gradient(circle at center, transparent 0%, rgba(2, 62, 138, 0.4) 100%);
    z-index: 2;
}

.hero-content {
    position: relative;
    z-index: 3;
    text-align: center;
    color: var(--text-white);
    padding: 0 20px;
    animation: fadeInUp 1s ease-out;
}

.hero-title {
    font-size: 4rem;
    font-weight: 800;
    margin-bottom: 0.5rem;
    letter-spacing: -1px;
    text-shadow: 0 2px 10px rgba(0, 0, 0, 0.2);
}

.hero-subtitle {
    font-size: 2rem;
    font-weight: 600;
    margin-bottom: 1.5rem;
    color: var(--accent-light);
}

.hero-tagline {
    font-size: 1.3rem;
    margin-bottom: 0.5rem;
    opacity: 0.95;
}

.hero-tagline-zh {
    font-size: 1.1rem;
    margin-bottom: 2rem;
    opacity: 0.9;
    font-weight: 300;
}

.cta-button {
    display: inline-block;
    padding: 1rem 2.5rem;
    background: var(--text-white);
    color: var(--primary-dark);
    text-decoration: none;
    border-radius: 50px;
    font-weight: 700;
    font-size: 1rem;
    transition: all 0.3s ease;
    box-shadow: 0 4px 15px rgba(0, 0, 0, 0.2);
}

.cta-button:hover {
    transform: translateY(-2px);
    box-shadow: 0 6px 20px rgba(0, 0, 0, 0.3);
    background: var(--accent-light);
}

.scroll-indicator {
    position: absolute;
    bottom: 30px;
    left: 50%;
    transform: translateX(-50%);
    z-index: 3;
    display: flex;
    gap: 8px;
}

.scroll-indicator span {
    width: 3px;
    height: 20px;
    background: var(--text-white);
    border-radius: 3px;
    animation: scroll 1.5s infinite;
}

.scroll-indicator span:nth-child(2) {
    animation-delay: 0.2s;
}

.scroll-indicator span:nth-child(3) {
    animation-delay: 0.4s;
}

@keyframes fadeInUp {
    from {
        opacity: 0;
        transform: translateY(30px);
    }
    to {
        opacity: 1;
        transform: translateY(0);
    }
}

@keyframes scroll {
    0%, 100% {
        opacity: 0.3;
        transform: scaleY(1);
    }
    50% {
        opacity: 1;
        transform: scaleY(1.5);
    }
}

/* ===== Section Styling ===== */
section {
    padding: var(--section-padding);
}

.section-header {
    text-align: center;
    margin-bottom: 4rem;
}

.section-title {
    font-size: 2.5rem;
    color: var(--primary-dark);
    margin-bottom: 0.5rem;
}

.section-subtitle {
    font-size: 1.2rem;
    color: var(--accent);
    margin-bottom: 1rem;
    font-weight: 500;
}

.section-divider {
    width: 80px;
    height: 4px;
    background: var(--gradient-2);
    margin: 1.5rem auto 0;
    border-radius: 2px;
}

/* ===== About Section ===== */
.about {
    background: var(--bg-white);
}

.about-content {
    max-width: 900px;
    margin: 0 auto;
}

.about-text p {
    font-size: 1.1rem;
    line-height: 1.8;
    margin-bottom: 1.5rem;
}

.lead-paragraph {
    font-size: 1.15rem !important;
    color: var(--text-medium);
    font-weight: 400;
}

.mission-highlight {
    background: linear-gradient(135deg, rgba(0, 180, 216, 0.05) 0%, rgba(46, 196, 182, 0.05) 100%);
    padding: 2.5rem;
    border-radius: 12px;
    border-left: 4px solid var(--accent);
    margin-top: 2rem;
}

.mission-highlight h3 {
    color: var(--primary-dark);
    font-size: 1.5rem;
    margin-bottom: 1rem;
}

/* About Sections */
.about-sections {
    margin-top: 3rem;
    display: grid;
    gap: 2.5rem;
}

.about-section {
    background: var(--bg-white);
    padding: 2rem;
    border-radius: 12px;
    border: 1px solid var(--bg-gray);
    transition: all 0.3s ease;
}

.about-section:hover {
    box-shadow: var(--shadow-md);
    border-color: var(--accent-light);
}

.about-section h3 {
    color: var(--primary-dark);
    font-size: 1.4rem;
    margin-bottom: 0.5rem;
}

.about-section > p {
    color: var(--text-medium);
    margin-bottom: 1.5rem;
}

.about-section > p.zh-text {
    color: var(--text-light);
    font-size: 0.95rem;
    margin-bottom: 1.5rem;
}

/* Tech List */
.tech-list,
.talent-list {
    list-style: none;
    padding: 0;
}

.tech-list li,
.talent-list li {
    padding: 1.2rem 0;
    border-bottom: 1px solid var(--bg-gray);
    color: var(--text-medium);
    line-height: 1.7;
}

.tech-list li:last-child,
.talent-list li:last-child {
    border-bottom: none;
    padding-bottom: 0;
}

.tech-list li strong,
.talent-list li strong {
    color: var(--primary-dark);
    font-size: 1.05rem;
}

.tech-list li .zh-text,
.talent-list li .zh-text {
    display: block;
    margin-top: 0.5rem;
    color: var(--text-light);
    font-size: 0.9rem;
}

/* Field Tags */
.field-tags {
    display: flex;
    flex-wrap: wrap;
    gap: 1rem;
    margin-top: 1rem;
}

.field-tag {
    background: linear-gradient(135deg, rgba(0, 180, 216, 0.1) 0%, rgba(46, 196, 182, 0.1) 100%);
    color: var(--primary-dark);
    padding: 0.6rem 1.2rem;
    border-radius: 25px;
    font-size: 0.9rem;
    font-weight: 500;
    border: 1px solid var(--accent-light);
    transition: all 0.3s ease;
}

.field-tag:hover {
    background: var(--gradient-2);
    color: white;
    transform: translateY(-2px);
    box-shadow: 0 4px 12px rgba(0, 180, 216, 0.2);
}

.field-tag .zh-text {
    display: block;
    font-size: 0.8rem;
    font-weight: 400;
    opacity: 0.8;
    margin-top: 0.2rem;
}

/* ===== Leader Section ===== */
.leader {
    background: var(--bg-light);
}

.leader-content {
    display: grid;
    grid-template-columns: 300px 1fr;
    gap: 3rem;
    align-items: start;
    max-width: 1000px;
    margin: 0 auto;
}

.leader-image {
    position: relative;
    align-self: start;
    padding-top: 0.5rem;
}

.leader-photo {
    width: 100%;
    aspect-ratio: 3/4;
    border-radius: 12px;
    overflow: hidden;
    box-shadow: var(--shadow-lg);
    background: var(--bg-gray);
}

.leader-photo img {
    width: 100%;
    height: 100%;
    object-fit: cover;
}

.leader-info {
    padding: 0;
}

.leader-name {
    font-size: 2rem;
    color: var(--primary-dark);
    margin-bottom: 0.3rem;
}

.leader-title {
    font-size: 1.1rem;
    color: var(--accent);
    font-weight: 600;
    margin-bottom: 0.5rem;
}

.leader-title-zh {
    font-size: 1rem;
    color: var(--text-light);
    margin-bottom: 2rem;
}

.leader-positions {
    margin-bottom: 2rem;
}

.positions-card {
    background: var(--bg-white);
    border-radius: 8px;
    border-left: 3px solid var(--accent);
    padding: 1.5rem;
    box-shadow: var(--shadow-sm);
}

.position-row {
    padding: 0.8rem 0;
    border-bottom: 1px solid var(--bg-gray);
    line-height: 1.6;
}

.position-row:last-child {
    border-bottom: none;
    padding-bottom: 0;
}

.position-row strong {
    display: block;
    color: var(--primary-dark);
    font-size: 1rem;
    margin-bottom: 0.2rem;
}

.position-row span {
    display: block;
    color: var(--text-medium);
    font-size: 0.9rem;
    margin-bottom: 0.15rem;
}

.position-row em {
    display: block;
    color: var(--accent);
    font-size: 0.85rem;
    font-style: normal;
    font-weight: 500;
}

.leader-bio h4 {
    font-size: 1rem;
    color: var(--primary);
    margin-bottom: 1rem;
    text-transform: uppercase;
    letter-spacing: 1px;
}

.leader-bio p {
    color: var(--text-medium);
    line-height: 1.8;
    margin-bottom: 1rem;
}

.leader-awards {
    margin-top: 2rem;
}

.leader-awards h4 {
    font-size: 1rem;
    color: var(--primary);
    margin-bottom: 1rem;
    text-transform: uppercase;
    letter-spacing: 1px;
}

.awards-list {
    list-style: none;
    padding: 0;
}

.awards-list li {
    padding: 0.8rem 0;
    padding-left: 1.5rem;
    position: relative;
    color: var(--text-medium);
    font-size: 0.95rem;
    line-height: 1.6;
}

.awards-list li::before {
    content: '🏆';
    position: absolute;
    left: 0;
    top: 0.8rem;
    font-size: 0.9rem;
}

/* ===== Quotes ===== */
.quote {
    background: var(--bg-light);
    padding: 2rem 2.5rem;
    border-radius: 12px;
    border-left: 4px solid var(--primary);
    margin: 2rem 0 3rem;
    font-style: italic;
    font-size: 1.2rem;
    color: var(--text-medium);
    position: relative;
}

.quote::before {
    content: '"';
    position: absolute;
    top: -10px;
    left: 20px;
    font-size: 4rem;
    color: var(--accent-light);
    opacity: 0.5;
    font-family: Georgia, serif;
}

.quote footer {
    display: block;
    margin-top: 1rem;
    font-size: 1rem;
    color: var(--text-light);
    font-style: normal;
}

/* ===== Research Section ===== */
.research {
    background: var(--bg-light);
}

.research-intro {
    max-width: 900px;
    margin: 0 auto 3rem;
}

.research-grid {
    display: grid;
    grid-template-columns: repeat(2, 1fr);
    gap: 2rem;
    margin-top: 3rem;
}

.research-card {
    background: var(--bg-white);
    padding: 2.5rem;
    border-radius: 12px;
    box-shadow: var(--shadow-sm);
    transition: all 0.3s ease;
    position: relative;
    overflow: hidden;
}

.research-card::before {
    content: '';
    position: absolute;
    top: 0;
    left: 0;
    width: 100%;
    height: 4px;
    background: var(--gradient-2);
    transform: scaleX(0);
    transition: transform 0.3s ease;
}

.research-card:hover {
    transform: translateY(-5px);
    box-shadow: var(--shadow-lg);
}

.research-card:hover::before {
    transform: scaleX(1);
}

.research-card.full-width {
    grid-column: 1 / -1;
}

.research-card h3 {
    font-size: 1.5rem;
    color: var(--primary-dark);
    margin-bottom: 0.5rem;
}

.research-card p {
    color: var(--text-medium);
    line-height: 1.7;
}

.card-icon {
    width: 60px;
    height: 60px;
    margin-bottom: 1.5rem;
    color: var(--primary);
}

.card-icon svg {
    width: 100%;
    height: 100%;
}

.card-icon.electrical {
    color: var(--primary-light);
}

.card-icon.chemical {
    color: var(--accent);
}

.featured-badge {
    position: absolute;
    top: 1.5rem;
    right: 1.5rem;
    background: var(--gradient-2);
    color: var(--text-white);
    padding: 0.5rem 1rem;
    border-radius: 20px;
    font-size: 0.8rem;
    font-weight: 700;
    text-transform: uppercase;
    letter-spacing: 0.5px;
}

.research-card.featured {
    border: 2px solid var(--accent);
}

/* ===== Technology Section ===== */
.technology {
    background: var(--bg-white);
}

.tech-intro {
    max-width: 900px;
    margin: 0 auto 3rem;
}

.technology-grid {
    display: grid;
    gap: 2rem;
    margin-top: 3rem;
}

.tech-card {
    background: var(--bg-white);
    border: 1px solid var(--bg-gray);
    border-radius: 12px;
    overflow: hidden;
    transition: all 0.3s ease;
    display: grid;
    grid-template-columns: 1fr 1.5fr;
    gap: 0;
}

.tech-card:hover {
    box-shadow: var(--shadow-md);
    border-color: var(--accent-light);
    transform: translateY(-5px);
}

.tech-card.core-tech {
    display: grid;
    grid-template-columns: 1fr 2fr;
    gap: 2rem;
    align-items: center;
    background: linear-gradient(135deg, rgba(0, 180, 216, 0.03) 0%, rgba(46, 196, 182, 0.03) 100%);
    border: 2px solid var(--primary-light);
    padding: 2.5rem;
}

.tech-visual {
    display: flex;
    justify-content: center;
    align-items: center;
}

.nanopore-animation {
    width: 150px;
    height: 150px;
    position: relative;
}

.nanopore {
    width: 80px;
    height: 80px;
    border: 3px solid var(--primary);
    border-radius: 50%;
    position: absolute;
    top: 50%;
    left: 50%;
    transform: translate(-50%, -50%);
    animation: pulse 2s infinite;
}

.molecules {
    position: absolute;
    width: 100%;
    height: 100%;
}

.molecule {
    position: absolute;
    width: 12px;
    height: 12px;
    background: var(--accent);
    border-radius: 50%;
    animation: float 3s infinite;
}

.molecule:nth-child(1) {
    top: 20%;
    left: 30%;
    animation-delay: 0s;
}

.molecule:nth-child(2) {
    top: 60%;
    right: 20%;
    animation-delay: 1s;
}

.molecule:nth-child(3) {
    bottom: 20%;
    left: 50%;
    animation-delay: 2s;
}

@keyframes pulse {
    0%, 100% {
        transform: translate(-50%, -50%) scale(1);
        opacity: 1;
    }
    50% {
        transform: translate(-50%, -50%) scale(1.1);
        opacity: 0.8;
    }
}

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

.tech-content h3 {
    font-size: 1.8rem;
    color: var(--primary-dark);
    margin-bottom: 0.5rem;
}

.tech-image {
    width: 100%;
    height: 100%;
    min-height: 350px;
    display: flex;
    align-items: center;
    justify-content: center;
    background: var(--bg-light);
    border-right: 1px solid var(--bg-gray);
}

.tech-image img {
    width: 100%;
    height: 100%;
    object-fit: contain;
    padding: 1.5rem;
    transition: transform 0.3s ease;
}

.tech-card:hover .tech-image img {
    transform: scale(1.02);
}

.tech-card-content {
    padding: 2.5rem;
    display: flex;
    flex-direction: column;
    justify-content: center;
}

.tech-card-content h3 {
    font-size: 1.6rem;
    color: var(--primary-dark);
    margin-bottom: 0.5rem;
}

.tech-card-content p {
    color: var(--text-medium);
    line-height: 1.7;
}

.tech-icon {
    width: 60px;
    height: 60px;
    margin-bottom: 1.5rem;
    color: var(--accent);
}

.tech-icon svg {
    width: 100%;
    height: 100%;
}

/* ===== Ecosystem Section ===== */
.ecosystem {
    background: var(--bg-light);
}

.ecosystem-intro {
    max-width: 900px;
    margin: 0 auto 3rem;
}

.ecosystem-grid {
    display: grid;
    grid-template-columns: repeat(3, 1fr);
    gap: 2rem;
    margin-bottom: 4rem;
}

.eco-card {
    background: var(--bg-white);
    padding: 2.5rem 2rem;
    border-radius: 12px;
    box-shadow: var(--shadow-sm);
    text-align: center;
    transition: all 0.3s ease;
    position: relative;
    overflow: hidden;
}

.eco-icon {
    font-size: 3rem;
    margin-bottom: 1.5rem;
}

.eco-card h3 {
    font-size: 1.3rem;
    color: var(--primary-dark);
    margin-bottom: 0.3rem;
}

.eco-subtitle {
    color: var(--accent);
    font-weight: 600;
    margin-bottom: 1rem;
    font-size: 0.95rem;
}

.eco-card h4 {
    font-size: 1.1rem;
    color: var(--text-dark);
    margin-bottom: 0.5rem;
}

.eco-card > p:not(.eco-subtitle):not(.zh-text) {
    color: var(--text-medium);
    font-size: 0.95rem;
    text-align: left;
}

.eco-card::before {
    content: '';
    position: absolute;
    top: 0;
    left: 0;
    width: 100%;
    height: 4px;
}

.eco-card.academic::before {
    background: var(--primary);
}

.eco-card.engineering::before {
    background: var(--accent);
}

.eco-card.strategic::before {
    background: var(--primary-dark);
}

.eco-card:hover {
    transform: translateY(-5px);
    box-shadow: var(--shadow-lg);
}

/* ===== Join Section ===== */
.join {
    background: var(--bg-white);
}

.join-intro {
    max-width: 900px;
    margin: 0 auto 3rem;
}

.join-content {
    max-width: 900px;
    margin: 0 auto 3rem;
    text-align: left;
}

.join-lead {
    font-size: 1.15rem;
    color: var(--text-medium);
    line-height: 1.8;
    margin-bottom: 1.5rem;
}

.positions-grid {
    display: grid;
    grid-template-columns: repeat(2, 1fr);
    gap: 2rem;
    margin-bottom: 3rem;
}

.position-card {
    background: linear-gradient(135deg, rgba(0, 180, 216, 0.05) 0%, rgba(46, 196, 182, 0.05) 100%);
    padding: 2.5rem;
    border-radius: 12px;
    border: 1px solid var(--accent-light);
}

.position-card h3 {
    font-size: 1.5rem;
    color: var(--primary-dark);
    margin-bottom: 0.5rem;
}

.positions-list {
    list-style: none;
    margin-top: 1.5rem;
}

.positions-list li {
    padding: 0.8rem 0;
    border-bottom: 1px solid var(--bg-gray);
    color: var(--text-medium);
    display: flex;
    align-items: center;
}

.positions-list li .zh-text {
    margin-left: 0.5rem;
}

.positions-list li::before {
    content: '▸';
    color: var(--accent);
    margin-right: 10px;
    font-size: 1.2rem;
}

.positions-list li:last-child {
    border-bottom: none;
}

.contact-cta {
    text-align: center;
    padding: 3rem;
    background: var(--gradient-1);
    border-radius: 12px;
    color: var(--text-white);
}

.contact-cta h3 {
    font-size: 2rem;
    margin-bottom: 0.5rem;
    color: var(--text-white);
}

.cta-button.large {
    display: inline-block;
    margin: 1.5rem 0;
    padding: 1.2rem 3rem;
    font-size: 1.1rem;
    background: var(--text-white);
    color: var(--primary-dark);
}

.contact-note {
    font-size: 0.9rem;
    opacity: 0.9;
    margin-top: 1rem;
}

/* ===== Footer ===== */
.footer {
    background: var(--primary-dark);
    color: var(--text-white);
    padding: 3rem 0 1.5rem;
}

.footer-content {
    display: grid;
    grid-template-columns: 1fr 2fr 1fr;
    gap: 3rem;
    margin-bottom: 2rem;
}

.footer-logo img {
    height: 80px;
    width: auto;
    opacity: 0.9;
}

.footer-info h3 {
    font-size: 1.5rem;
    margin-bottom: 0.5rem;
}

.footer-tagline {
    opacity: 0.9;
    font-size: 0.95rem;
    margin-bottom: 0.5rem;
    font-style: italic;
}

.footer-address {
    margin-top: 0.8rem;
    opacity: 0.8;
    font-size: 0.9rem;
}

.footer-links {
    display: grid;
    grid-template-columns: repeat(2, 1fr);
    gap: 0.8rem 2rem;
}

.footer-links a {
    color: var(--text-white);
    text-decoration: none;
    opacity: 0.9;
    transition: opacity 0.3s ease;
}

.footer-links a:hover {
    opacity: 1;
}

.footer-bottom {
    text-align: center;
    padding-top: 2rem;
    border-top: 1px solid rgba(255, 255, 255, 0.1);
}

.footer-bottom p {
    opacity: 0.7;
    font-size: 0.9rem;
}

/* ===== Challenges Section ===== */
.challenges {
    background: var(--bg-light);
}

.challenges-intro {
    max-width: 900px;
    margin: 0 auto 3rem;
}

.challenges-grid {
    display: grid;
    grid-template-columns: repeat(auto-fit, minmax(350px, 1fr));
    gap: 2rem;
    margin-top: 3rem;
}

.challenge-card {
    background: var(--bg-white);
    padding: 2.5rem 2rem;
    border-radius: 12px;
    box-shadow: var(--shadow-sm);
    transition: all 0.3s ease;
    position: relative;
    overflow: hidden;
    border: 1px solid var(--bg-gray);
}

.challenge-card::before {
    content: '';
    position: absolute;
    top: 0;
    left: 0;
    width: 100%;
    height: 4px;
    background: var(--gradient-2);
    transform: scaleX(0);
    transition: transform 0.3s ease;
}

.challenge-card:hover {
    transform: translateY(-5px);
    box-shadow: var(--shadow-lg);
    border-color: var(--accent-light);
}

.challenge-card:hover::before {
    transform: scaleX(1);
}

.challenge-number {
    position: absolute;
    top: 1.5rem;
    right: 1.5rem;
    font-size: 3rem;
    font-weight: 800;
    color: var(--accent-light);
    opacity: 0.3;
    line-height: 1;
}

.challenge-card h3 {
    font-size: 1.4rem;
    color: var(--primary-dark);
    margin-bottom: 0.5rem;
    padding-right: 3rem;
}

.challenge-card > p {
    color: var(--text-medium);
    line-height: 1.7;
    margin-bottom: 1rem;
}

.challenge-card > p.zh-text {
    color: var(--text-light);
    font-size: 0.95rem;
    margin-bottom: 1.5rem;
}

/* ===== Scroll Progress Indicator ===== */
.scroll-progress {
    position: fixed;
    top: 0;
    left: 0;
    width: 0%;
    height: 3px;
    background: var(--gradient-2);
    z-index: 1001;
    transition: width 0.1s ease-out;
}

/* ===== Back to Top Button ===== */
.back-to-top {
    position: fixed;
    bottom: 30px;
    right: 30px;
    width: 50px;
    height: 50px;
    background: var(--gradient-2);
    border: none;
    border-radius: 50%;
    color: var(--text-white);
    cursor: pointer;
    opacity: 0;
    visibility: hidden;
    transition: all 0.3s ease;
    z-index: 1000;
    box-shadow: var(--shadow-md);
    display: flex;
    align-items: center;
    justify-content: center;
}

.back-to-top svg {
    width: 24px;
    height: 24px;
}

.back-to-top:hover {
    transform: translateY(-3px);
    box-shadow: var(--shadow-lg);
}

.back-to-top.visible {
    opacity: 1;
    visibility: visible;
}

.back-to-top:active {
    transform: translateY(-1px);
}