/* ============================================
   CSS Custom Properties (Light & Dark Mode)
   ============================================ */
:root {
    --primary-color: #4A90A4;
    --primary-light: #87CEEB;
    --text-color: #2C3E50;
    --text-secondary: #5D6D7E;
    --background-color: #ffffff;
    --secondary-background: #F8FAFC;
    --accent-color: #3D7A8C;
    --border-color: #E2E8F0;
    --hover-color: #E0F2FE;
    --card-shadow: 0 4px 6px rgba(0, 0, 0, 0.04);
    --card-shadow-hover: 0 10px 25px rgba(0, 0, 0, 0.08);
    --transition-speed: 0.3s;
}

/* Dark Mode Variables */
[data-theme="dark"] {
    --primary-color: #87CEEB;
    --primary-light: #B0E0E6;
    --text-color: #E2E8F0;
    --text-secondary: #94A3B8;
    --background-color: #0F172A;
    --secondary-background: #1E293B;
    --accent-color: #7DD3FC;
    --border-color: #334155;
    --hover-color: #1E3A5F;
    --card-shadow: 0 4px 6px rgba(0, 0, 0, 0.3);
    --card-shadow-hover: 0 10px 25px rgba(0, 0, 0, 0.4);
}

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

html {
    scroll-behavior: smooth;
}

body {
    font-family: 'Inter', sans-serif;
    line-height: 1.6;
    color: var(--text-color);
    background-color: var(--background-color);
    transition: background-color var(--transition-speed) ease, color var(--transition-speed) ease;
}

.container {
    max-width: 1200px;
    margin: 0 auto;
    padding: 0 2rem;
}

/* ============================================
   Skip Link (Accessibility)
   ============================================ */
.skip-link {
    position: absolute;
    top: -100%;
    left: 50%;
    transform: translateX(-50%);
    background: var(--primary-color);
    color: white;
    padding: 0.75rem 1.5rem;
    border-radius: 0 0 8px 8px;
    z-index: 9999;
    transition: top 0.3s ease;
    text-decoration: none;
    font-weight: 500;
}

.skip-link:focus {
    top: 0;
}

/* ============================================
   Header and Navigation
   ============================================ */
header {
    background-color: var(--background-color);
    backdrop-filter: blur(10px);
    box-shadow: var(--card-shadow);
    position: fixed;
    width: 100%;
    top: 0;
    z-index: 1000;
    transition: all var(--transition-speed) ease;
}

header:hover {
    box-shadow: var(--card-shadow-hover);
}

nav {
    padding: 1rem 2rem;
    display: flex;
    justify-content: center;
    align-items: center;
    position: relative;
    max-width: 1200px;
    margin: 0 auto;
    min-height: 60px;
}

/* Nav Logo - Hidden by default, shown on scroll */
.nav-logo {
    position: absolute;
    left: 2rem;
    font-size: 1.25rem;
    font-weight: 600;
    color: var(--text-color);
    text-decoration: none;
    opacity: 0;
    transform: translateY(-10px);
    pointer-events: none;
    transition: all var(--transition-speed) ease;
}

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

.nav-logo::after {
    display: none;
}

header.scrolled .nav-logo {
    opacity: 1;
    transform: translateY(0);
    pointer-events: auto;
}

.nav-menu {
    display: flex;
    justify-content: center;
    list-style: none;
    gap: 1.5rem;
}

nav a {
    text-decoration: none;
    color: var(--text-color);
    font-weight: 500;
    transition: all var(--transition-speed) ease;
    position: relative;
    padding: 0.5rem 0;
    font-size: 0.95rem;
}

nav a::after {
    content: '';
    position: absolute;
    width: 0;
    height: 2px;
    bottom: 0;
    left: 0;
    background-color: var(--primary-color);
    transition: width var(--transition-speed) ease;
}

nav a:hover::after {
    width: 100%;
}

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

nav a i {
    margin-right: 0.4rem;
}

/* Mobile Menu Toggle */
.menu-toggle {
    display: none;
    flex-direction: column;
    justify-content: center;
    align-items: center;
    gap: 5px;
    background: none;
    border: none;
    cursor: pointer;
    padding: 8px;
    z-index: 1001;
    width: 40px;
    height: 40px;
}

.hamburger-line {
    display: block;
    width: 22px;
    height: 2px;
    background-color: var(--text-color);
    transition: all var(--transition-speed) ease;
}

/* Hamburger Animation */
.menu-toggle.active .hamburger-line:nth-child(1) {
    transform: translateY(7px) rotate(45deg);
}

.menu-toggle.active .hamburger-line:nth-child(2) {
    opacity: 0;
}

.menu-toggle.active .hamburger-line:nth-child(3) {
    transform: translateY(-7px) rotate(-45deg);
}

/* Theme Toggle */
.theme-toggle {
    position: absolute;
    right: 2rem;
    background: none;
    border: 2px solid var(--border-color);
    cursor: pointer;
    padding: 0.5rem;
    border-radius: 50%;
    display: flex;
    align-items: center;
    justify-content: center;
    transition: all var(--transition-speed) ease;
    color: var(--text-color);
    width: 38px;
    height: 38px;
    flex-shrink: 0;
}

.theme-toggle:hover {
    border-color: var(--primary-color);
    background-color: var(--hover-color);
    color: var(--primary-color);
}

.theme-toggle i {
    font-size: 0.95rem;
}

/* ============================================
   Hero Section
   ============================================ */
#hero {
    padding: 10rem 0 5rem;
    background: linear-gradient(135deg, var(--secondary-background) 0%, var(--background-color) 100%);
    position: relative;
    overflow: hidden;
}

#hero::before {
    content: '';
    position: absolute;
    top: 0;
    left: 0;
    right: 0;
    bottom: 0;
    background: radial-gradient(circle at 30% 50%, rgba(74, 144, 164, 0.08) 0%, transparent 70%);
    pointer-events: none;
}

/* Hero Side-by-Side Layout */
.hero-content {
    display: flex;
    align-items: center;
    gap: 4rem;
}

.hero-image {
    flex-shrink: 0;
    overflow: hidden;
    border-radius: 50%;
}

.hero-image img {
    width: 260px;
    height: 260px;
    border-radius: 50%;
    object-fit: cover;
    border: 1px solid var(--border-color);
    box-shadow: var(--card-shadow);
}

.hero-text {
    text-align: left;
}

#hero h1 {
    font-size: 3.5rem;
    font-weight: 700;
    margin-bottom: 1rem;
    color: var(--text-color);
    letter-spacing: -0.02em;
}

#hero h2 {
    font-size: 1.5rem;
    font-weight: 500;
    color: var(--primary-color);
    margin-bottom: 1rem;
}

.hero-text p {
    color: var(--text-secondary);
    font-size: 1.1rem;
    max-width: 600px;
    margin-bottom: 2rem;
}

/* Hero CTA Buttons */
.hero-cta {
    display: flex;
    gap: 1rem;
    justify-content: flex-start;
    flex-wrap: wrap;
}

.btn {
    display: inline-flex;
    align-items: center;
    padding: 0.875rem 1.75rem;
    border-radius: 8px;
    font-weight: 500;
    text-decoration: none;
    transition: all var(--transition-speed) ease;
    font-size: 1rem;
}

.btn-primary {
    background-color: var(--primary-color);
    color: white;
}

.btn-primary:hover {
    background-color: var(--accent-color);
    transform: translateY(-2px);
    box-shadow: 0 4px 12px rgba(74, 144, 164, 0.3);
}

.btn-secondary {
    background-color: transparent;
    color: var(--primary-color);
    border: 2px solid var(--primary-color);
}

.btn-secondary:hover {
    background-color: var(--primary-color);
    color: white;
    transform: translateY(-2px);
}


/* ============================================
   Sections Common Styles
   ============================================ */
section {
    padding: 5rem 0;
    position: relative;
}

section:nth-child(even) {
    background-color: var(--secondary-background);
}

h2 {
    font-size: 2rem;
    margin-bottom: 2rem;
    color: var(--text-color);
    position: relative;
    display: inline-block;
}

h2::after {
    content: '';
    position: absolute;
    bottom: -0.5rem;
    left: 0;
    width: 3rem;
    height: 3px;
    background-color: var(--primary-color);
    transition: width var(--transition-speed) ease;
}

h2:hover::after {
    width: 100%;
}

h2 i, h3 i {
    margin-right: 0.5rem;
    color: var(--primary-color);
}

/* ============================================
   Research Grid
   ============================================ */
.research-grid {
    display: grid;
    grid-template-columns: repeat(auto-fit, minmax(250px, 1fr));
    gap: 2rem;
    margin-top: 2rem;
}

.research-item {
    background-color: var(--background-color);
    padding: 2rem;
    border-radius: 12px;
    box-shadow: var(--card-shadow);
    transition: all 0.4s cubic-bezier(0.4, 0, 0.2, 1);
    border: 1px solid var(--border-color);
    text-align: center;
    transform-origin: center;
}

.research-item:hover {
    transform: translateY(-5px);
    box-shadow: var(--card-shadow-hover);
    border-color: var(--primary-color);
}

.research-item h3 {
    color: var(--primary-color);
    margin-bottom: 1rem;
}

.research-item p {
    color: var(--text-secondary);
}

.research-icon {
    font-size: 2.5rem;
    color: var(--primary-color);
    margin-bottom: 1rem;
    transition: all 0.4s cubic-bezier(0.4, 0, 0.2, 1);
}

.research-item:hover .research-icon {
    color: var(--accent-color);
    transform: scale(1.1);
}

/* ============================================
   Projects Section
   ============================================ */
.projects-grid {
    display: grid;
    grid-template-columns: repeat(auto-fit, minmax(320px, 1fr));
    gap: 2rem;
    margin-top: 2rem;
}

.project-card {
    background-color: var(--background-color);
    padding: 2rem;
    border-radius: 12px;
    box-shadow: var(--card-shadow);
    border: 1px solid var(--border-color);
    transition: all 0.4s cubic-bezier(0.4, 0, 0.2, 1);
    position: relative;
    overflow: hidden;
}

.project-card::before {
    content: '';
    position: absolute;
    top: 0;
    left: 0;
    width: 100%;
    height: 4px;
    background: linear-gradient(90deg, var(--primary-color), var(--accent-color));
    transform: scaleX(0);
    transform-origin: left;
    transition: transform var(--transition-speed) ease;
}

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

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

.project-icon {
    font-size: 2.5rem;
    color: var(--primary-color);
    margin-bottom: 1rem;
}

.project-card h3 {
    color: var(--text-color);
    margin-bottom: 0.75rem;
    font-size: 1.25rem;
}

.project-card p {
    color: var(--text-secondary);
    margin-bottom: 1.25rem;
    line-height: 1.7;
}

.project-tags {
    display: flex;
    flex-wrap: wrap;
    gap: 0.5rem;
}

.project-tags span {
    background-color: var(--secondary-background);
    color: var(--text-secondary);
    padding: 0.35rem 0.75rem;
    border-radius: 20px;
    font-size: 0.8rem;
    font-weight: 500;
    border: 1px solid var(--border-color);
}

/* ============================================
   Publications
   ============================================ */
.publication {
    margin-bottom: 2rem;
}

.publication h3 {
    margin-bottom: 1rem;
}

.publication ul {
    list-style: none;
}

.publication li {
    margin-bottom: 1.5rem;
    padding: 1.5rem;
    background-color: var(--background-color);
    border-radius: 12px;
    box-shadow: var(--card-shadow);
    transition: all var(--transition-speed) ease;
    border: 1px solid var(--border-color);
}

.publication li:hover {
    transform: translateX(5px);
    border-color: var(--primary-color);
    box-shadow: var(--card-shadow-hover);
}

.publication li i {
    color: var(--primary-color);
    margin-right: 0.5rem;
}

.publication li em {
    color: var(--text-secondary);
}

/* ============================================
   Education
   ============================================ */
.education-item {
    margin-bottom: 2rem;
    padding: 2rem;
    background-color: var(--background-color);
    border-radius: 12px;
    box-shadow: var(--card-shadow);
    transition: all 0.4s cubic-bezier(0.4, 0, 0.2, 1);
    border: 1px solid var(--border-color);
    text-align: left;
    transform-origin: center;
}

.education-item:hover {
    transform: translateX(5px);
    border-color: var(--primary-color);
    box-shadow: var(--card-shadow-hover);
}

.education-item h3 {
    color: var(--primary-color);
    margin-bottom: 1rem;
}

.education-item p {
    color: var(--text-secondary);
    margin-bottom: 0.5rem;
}

.education-item p i {
    color: var(--primary-color);
    width: 1.5rem;
    text-align: center;
    margin-right: 0.5rem;
}

.education-icon {
    font-size: 2.5rem;
    color: var(--primary-color);
    margin-bottom: 1rem;
    transition: color 0.4s cubic-bezier(0.4, 0, 0.2, 1);
}

.education-item:hover .education-icon {
    color: var(--accent-color);
}

/* ============================================
   Contact Section
   ============================================ */
.contact-content {
    margin-top: 1rem;
}

.contact-info {
    margin-bottom: 1.5rem;
}

.contact-info p {
    margin-bottom: 0.75rem;
    color: var(--text-secondary);
}

#contact p i {
    color: var(--primary-color);
    width: 1.5rem;
    text-align: center;
    margin-right: 0.5rem;
}

.social-links {
    display: flex;
    gap: 1rem;
    flex-wrap: wrap;
}

.social-links a {
    color: var(--primary-color);
    text-decoration: none;
    font-weight: 500;
    transition: all var(--transition-speed) ease;
    padding: 0.75rem 1.25rem;
    border-radius: 8px;
    border: 1px solid var(--border-color);
    display: inline-flex;
    align-items: center;
    gap: 0.5rem;
}

.social-links a:hover {
    color: white;
    background-color: var(--primary-color);
    border-color: var(--primary-color);
    transform: translateY(-2px);
}

.social-links a i {
    font-size: 1.1rem;
}

/* ============================================
   Footer
   ============================================ */
footer {
    background-color: var(--secondary-background);
    padding: 2rem 0;
    text-align: center;
    margin-top: 0;
    border-top: 1px solid var(--border-color);
}

footer p {
    color: var(--text-secondary);
}

footer p i {
    margin-right: 0.5rem;
}

/* ============================================
   Back to Top Button
   ============================================ */
.back-to-top {
    position: fixed;
    bottom: 2rem;
    right: 2rem;
    width: 48px;
    height: 48px;
    background-color: var(--primary-color);
    color: white;
    border: none;
    border-radius: 50%;
    cursor: pointer;
    display: flex;
    align-items: center;
    justify-content: center;
    font-size: 1.25rem;
    box-shadow: 0 4px 12px rgba(74, 144, 164, 0.3);
    opacity: 0;
    visibility: hidden;
    transform: translateY(20px);
    transition: all var(--transition-speed) ease;
    z-index: 999;
}

.back-to-top.visible {
    opacity: 1;
    visibility: visible;
    transform: translateY(0);
}

.back-to-top:hover {
    background-color: var(--accent-color);
    transform: translateY(-3px);
    box-shadow: 0 6px 16px rgba(74, 144, 164, 0.4);
}

/* ============================================
   Animations
   ============================================ */
@keyframes float {
    0% {
        transform: translateY(0px);
    }
    50% {
        transform: translateY(-10px);
    }
    100% {
        transform: translateY(0px);
    }
}

/* ============================================
   Responsive Design
   ============================================ */
@media (max-width: 768px) {
    nav {
        padding: 0.875rem 1rem;
        min-height: 56px;
    }

    .menu-toggle {
        display: flex;
        position: absolute;
        left: 1rem;
    }

    /* Nav logo - centered on mobile */
    .nav-logo {
        position: static;
        left: auto;
        order: 0;
        font-size: 1.1rem;
    }

    header.scrolled .nav-logo {
        opacity: 1;
        transform: translateY(0);
        pointer-events: auto;
    }

    .nav-menu {
        position: fixed;
        top: 0;
        left: -100%;
        width: 280px;
        height: 100vh;
        background-color: var(--background-color);
        flex-direction: column;
        align-items: flex-start;
        padding: 5rem 2rem 2rem;
        gap: 0;
        transition: left var(--transition-speed) ease;
        box-shadow: 2px 0 10px rgba(0, 0, 0, 0.1);
        z-index: 1000;
    }

    .nav-menu.active {
        left: 0;
    }

    .nav-menu li {
        width: 100%;
    }

    .nav-menu a {
        display: block;
        padding: 1rem 0;
        border-bottom: 1px solid var(--border-color);
        width: 100%;
    }

    .theme-toggle {
        position: absolute;
        right: 1rem;
        width: 36px;
        height: 36px;
        padding: 0.4rem;
    }

    .theme-toggle i {
        font-size: 0.9rem;
    }

    #hero {
        padding: 8rem 0 4rem;
    }

    .hero-content {
        flex-direction: column;
        gap: 2rem;
        text-align: center;
    }

    .hero-image img {
        width: 200px;
        height: 200px;
        transform: scale(1.3) translateY(10%);
    }

    .hero-text {
        text-align: center;
    }

    #hero h1 {
        font-size: 2.5rem;
    }

    #hero h2 {
        font-size: 1.25rem;
    }

    .hero-cta {
        flex-direction: column;
        align-items: center;
        justify-content: center;
    }

    .btn {
        width: 100%;
        max-width: 250px;
        justify-content: center;
    }

    .research-grid,
    .projects-grid {
        grid-template-columns: 1fr;
    }

    .container {
        padding: 0 1rem;
    }

    section {
        padding: 3rem 0;
    }

    .social-links {
        flex-direction: column;
    }

    .social-links a {
        width: 100%;
        justify-content: center;
    }

    .back-to-top {
        bottom: 1.5rem;
        right: 1.5rem;
        width: 44px;
        height: 44px;
    }
}

/* Overlay for mobile menu */
.nav-overlay {
    position: fixed;
    top: 0;
    left: 0;
    width: 100%;
    height: 100%;
    background-color: rgba(0, 0, 0, 0.5);
    opacity: 0;
    visibility: hidden;
    transition: all var(--transition-speed) ease;
    z-index: 999;
}

.nav-overlay.active {
    opacity: 1;
    visibility: visible;
}

/* ============================================
   Print Styles
   ============================================ */
@media print {
    header, .back-to-top, .hero-cta {
        display: none;
    }

    section {
        page-break-inside: avoid;
    }

    body {
        color: #000;
        background: #fff;
    }
}