:root {
    --primary: #0d5c63;
    --primary-light: #14919b;
    --primary-dark: #0a4a4f;
    --accent: #d4a03c;
    --accent-light: #e8c068;
    --accent-dark: #b8872e;
    --white: #ffffff;
    --gray-50: #f9fafb;
    --gray-100: #f3f4f6;
    --gray-200: #e5e7eb;
    --gray-300: #d1d5db;
    --gray-400: #9ca3af;
    --gray-500: #6b7280;
    --gray-600: #4b5563;
    --gray-700: #374151;
    --gray-800: #1f2937;
    --gray-900: #111827;
    --ibm-blue: #0062ff;
    --flatiron-green: #00a870;
    --font-heading: 'Playfair Display', Georgia, serif;
    --font-body: 'Inter', -apple-system, BlinkMacSystemFont, 'Segoe UI', Roboto, sans-serif;
    --shadow-sm: 0 1px 2px rgba(0, 0, 0, 0.05);
    --shadow: 0 4px 6px -1px rgba(0, 0, 0, 0.1), 0 2px 4px -1px rgba(0, 0, 0, 0.06);
    --shadow-lg: 0 10px 15px -3px rgba(0, 0, 0, 0.1), 0 4px 6px -2px rgba(0, 0, 0, 0.05);
    --shadow-xl: 0 20px 25px -5px rgba(0, 0, 0, 0.1), 0 10px 10px -5px rgba(0, 0, 0, 0.04);
    --radius: 8px;
    --radius-lg: 16px;
}

/* Theme: Teal & Gold */
[data-theme="teal"] {
    --primary: #0d5c63;
    --primary-light: #14919b;
    --primary-dark: #0a4a4f;
    --accent: #d4a03c;
    --accent-light: #e8c068;
    --accent-dark: #b8872e;
}

/* Theme: Purple & Pink */
[data-theme="purple"] {
    --primary: #4a306d;
    --primary-light: #6b4d8a;
    --primary-dark: #2e1f44;
    --accent: #e056a0;
    --accent-light: #f07dba;
    --accent-dark: #c43d85;
}

/* Theme: Forest Green & Amber */
[data-theme="forest"] {
    --primary: #2d5a3d;
    --primary-light: #3d7a52;
    --primary-dark: #1e3d2a;
    --accent: #e67e22;
    --accent-light: #f5a54a;
    --accent-dark: #c96a15;
}

/* Theme: Slate & Coral (Modern) */
[data-theme="slate"] {
    --primary: #334155;
    --primary-light: #475569;
    --primary-dark: #1e293b;
    --accent: #f43f5e;
    --accent-light: #fb7185;
    --accent-dark: #e11d48;
}

/* Theme: Warm Brown & Terracotta */
[data-theme="warm"] {
    --primary: #5c4033;
    --primary-light: #7a5a48;
    --primary-dark: #3d2a22;
    --accent: #c96442;
    --accent-light: #e08866;
    --accent-dark: #a84e32;
}

/* Theme Switcher */
.theme-switcher {
    position: fixed;
    bottom: 24px;
    right: 24px;
    z-index: 1001;
    background: var(--white);
    border-radius: 12px;
    box-shadow: var(--shadow-xl);
    padding: 16px;
    display: flex;
    flex-direction: column;
    gap: 8px;
}

.theme-switcher-label {
    font-size: 0.75rem;
    font-weight: 600;
    color: var(--gray-500);
    text-transform: uppercase;
    letter-spacing: 0.5px;
    margin-bottom: 4px;
}

.theme-btn {
    width: 36px;
    height: 36px;
    border-radius: 50%;
    border: 3px solid transparent;
    cursor: pointer;
    transition: all 0.2s;
}

.theme-btn:hover {
    transform: scale(1.1);
}

.theme-btn.active {
    border-color: var(--gray-800);
}

.theme-btn[data-theme="default"] { background: linear-gradient(135deg, #1e3a5f 50%, #e07b53 50%); }
.theme-btn[data-theme="teal"] { background: linear-gradient(135deg, #0d5c63 50%, #d4a03c 50%); }
.theme-btn[data-theme="purple"] { background: linear-gradient(135deg, #4a306d 50%, #e056a0 50%); }
.theme-btn[data-theme="forest"] { background: linear-gradient(135deg, #2d5a3d 50%, #e67e22 50%); }
.theme-btn[data-theme="slate"] { background: linear-gradient(135deg, #334155 50%, #f43f5e 50%); }
.theme-btn[data-theme="warm"] { background: linear-gradient(135deg, #5c4033 50%, #c96442 50%); }

.theme-buttons {
    display: grid;
    grid-template-columns: repeat(3, 1fr);
    gap: 8px;
}

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

html {
    scroll-behavior: smooth;
}

body {
    font-family: var(--font-body);
    font-size: 16px;
    line-height: 1.6;
    color: var(--gray-800);
    background: var(--white);
}

.container {
    max-width: 1200px;
    margin: 0 auto;
    padding: 0 24px;
}

/* Navbar */
.navbar {
    position: fixed;
    top: 0;
    left: 0;
    right: 0;
    background: rgba(255, 255, 255, 0.95);
    backdrop-filter: blur(10px);
    z-index: 1000;
    border-bottom: 1px solid var(--gray-200);
}

.nav-container {
    max-width: 1200px;
    margin: 0 auto;
    padding: 0 24px;
    display: flex;
    align-items: center;
    justify-content: space-between;
    height: 72px;
}

.nav-logo {
    font-family: var(--font-heading);
    font-size: 1.5rem;
    font-weight: 700;
    color: var(--primary);
    text-decoration: none;
}

.nav-menu {
    display: flex;
    align-items: center;
    gap: 32px;
    list-style: none;
}

.nav-menu a {
    color: var(--gray-600);
    text-decoration: none;
    font-weight: 500;
    font-size: 0.95rem;
    transition: color 0.2s;
}

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

.nav-cta {
    background: var(--accent);
    color: var(--white) !important;
    padding: 10px 20px;
    border-radius: var(--radius);
    transition: background 0.2s !important;
}

.nav-cta:hover {
    background: var(--accent-dark) !important;
}

.nav-toggle {
    display: none;
    flex-direction: column;
    gap: 5px;
    background: none;
    border: none;
    cursor: pointer;
    padding: 5px;
}

.nav-toggle span {
    display: block;
    width: 25px;
    height: 2px;
    background: var(--gray-700);
    transition: all 0.3s;
}

/* Hero */
.hero {
    position: relative;
    min-height: 100vh;
    display: flex;
    align-items: center;
    justify-content: center;
    padding: 120px 24px 80px;
    overflow: hidden;
}

.hero-background {
    position: absolute;
    inset: 0;
    background: linear-gradient(135deg, var(--primary-dark) 0%, var(--primary) 50%, var(--primary-light) 100%);
    z-index: -2;
}

.hero-background::before {
    content: '';
    position: absolute;
    inset: 0;
    background: url("data:image/svg+xml,%3Csvg width='60' height='60' viewBox='0 0 60 60' xmlns='http://www.w3.org/2000/svg'%3E%3Cg fill='none' fill-rule='evenodd'%3E%3Cg fill='%23ffffff' fill-opacity='0.03'%3E%3Cpath d='M36 34v-4h-2v4h-4v2h4v4h2v-4h4v-2h-4zm0-30V0h-2v4h-4v2h4v4h2V6h4V4h-4zM6 34v-4H4v4H0v2h4v4h2v-4h4v-2H6zM6 4V0H4v4H0v2h4v4h2V6h4V4H6z'/%3E%3C/g%3E%3C/g%3E%3C/svg%3E");
    z-index: -1;
}

.hero-content {
    text-align: center;
    max-width: 900px;
    color: var(--white);
}

.hero-logos {
    display: flex;
    align-items: center;
    justify-content: center;
    gap: 16px;
    margin-bottom: 40px;
}

.hero-logos img.logo-ibm {
    height: 45px;
    filter: brightness(0) invert(1);
}

.hero-logos img.logo-flatiron {
    height: 45px;
    width: auto;
}

.logo-separator {
    font-size: 1.5rem;
    opacity: 0.5;
    font-weight: 300;
    line-height: 1;
    margin: 0 8px;
}

.logo-separator {
    font-size: 1.5rem;
    opacity: 0.5;
}

.hero h1 {
    font-family: var(--font-heading);
    font-size: clamp(2.5rem, 5vw, 4rem);
    font-weight: 700;
    line-height: 1.15;
    margin-bottom: 16px;
}

.hero-subtitle {
    font-size: clamp(1.25rem, 2.5vw, 1.75rem);
    opacity: 0.9;
    margin-bottom: 40px;
}

.hero-details {
    display: flex;
    flex-wrap: wrap;
    justify-content: center;
    gap: 32px;
    margin-bottom: 48px;
}

.detail-item {
    display: flex;
    align-items: center;
    gap: 10px;
    font-size: 1.1rem;
}

.detail-item svg {
    opacity: 0.8;
}

.hero-cta {
    display: inline-block;
    background: var(--accent);
    color: var(--white);
    padding: 16px 40px;
    border-radius: var(--radius);
    font-weight: 600;
    font-size: 1.1rem;
    text-decoration: none;
    transition: all 0.2s;
    box-shadow: var(--shadow-lg);
}

.hero-cta:hover {
    background: var(--accent-dark);
    transform: translateY(-2px);
    box-shadow: var(--shadow-xl);
}

/* Sections */
.section {
    padding: 100px 0;
}

.section-title {
    font-family: var(--font-heading);
    font-size: clamp(2rem, 4vw, 2.75rem);
    font-weight: 700;
    color: var(--primary);
    text-align: center;
    margin-bottom: 60px;
}

/* About */
.about {
    background: var(--gray-50);
}

.about-content {
    max-width: 800px;
    margin: 0 auto;
    font-size: 1.15rem;
    line-height: 1.8;
    color: var(--gray-700);
}

.about-content p {
    margin-bottom: 20px;
}

/* Topics */
.topics-grid {
    display: grid;
    grid-template-columns: repeat(auto-fit, minmax(300px, 1fr));
    gap: 24px;
}

.topic-card {
    background: var(--white);
    border: 1px solid var(--gray-200);
    border-radius: var(--radius-lg);
    padding: 32px;
    transition: all 0.3s;
}

.topic-card:hover {
    transform: translateY(-4px);
    box-shadow: var(--shadow-lg);
    border-color: var(--accent);
}

.topic-icon {
    width: 64px;
    height: 64px;
    background: linear-gradient(135deg, var(--primary) 0%, var(--primary-light) 100%);
    border-radius: 12px;
    display: flex;
    align-items: center;
    justify-content: center;
    margin-bottom: 20px;
    color: var(--white);
}

.topic-card h3 {
    font-size: 1.25rem;
    font-weight: 600;
    color: var(--gray-800);
    margin-bottom: 12px;
}

.topic-card p {
    color: var(--gray-600);
    font-size: 0.95rem;
}

/* Schedule */
.schedule {
    background: var(--gray-50);
}

.schedule-tabs {
    display: flex;
    justify-content: center;
    gap: 16px;
    margin-bottom: 40px;
}

.tab-btn {
    display: flex;
    flex-direction: column;
    align-items: center;
    padding: 16px 32px;
    background: var(--white);
    border: 2px solid var(--gray-200);
    border-radius: var(--radius);
    cursor: pointer;
    transition: all 0.2s;
}

.tab-btn:hover {
    border-color: var(--primary-light);
}

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

.tab-day {
    font-weight: 600;
    font-size: 1rem;
}

.tab-date {
    font-size: 0.875rem;
    opacity: 0.8;
}

.tab-content {
    display: none;
    background: var(--white);
    border-radius: var(--radius-lg);
    padding: 40px;
    box-shadow: var(--shadow);
}

.tab-content.active {
    display: block;
}

.venue-badge {
    display: inline-flex;
    align-items: center;
    gap: 8px;
    background: var(--primary);
    color: var(--white);
    padding: 8px 16px;
    border-radius: 20px;
    font-size: 0.875rem;
    font-weight: 500;
    margin-bottom: 32px;
}

.venue-badge.ibm {
    background: var(--ibm-blue);
}

.session-block {
    margin-bottom: 32px;
}

.session-title {
    font-size: 1.25rem;
    font-weight: 600;
    color: var(--gray-800);
    margin-bottom: 20px;
    padding-bottom: 12px;
    border-bottom: 2px solid var(--gray-200);
}

.time-slot {
    display: flex;
    align-items: center;
    gap: 16px;
    padding: 12px 0;
    border-bottom: 1px solid var(--gray-100);
}

.time-slot .time {
    font-weight: 600;
    color: var(--primary);
    min-width: 100px;
}

.time-slot .event {
    color: var(--gray-600);
}

.time-slot.lunch,
.time-slot.poster,
.time-slot.closing {
    background: var(--gray-50);
    padding: 16px;
    border-radius: var(--radius);
    margin: 20px 0;
    border: none;
}

.time-slot.poster {
    background: linear-gradient(135deg, var(--accent) 0%, var(--accent-light) 100%);
    color: var(--white);
}

.time-slot.poster .time {
    color: var(--white);
}

.time-slot.poster .event {
    color: var(--white);
    font-weight: 500;
}

.speakers-grid {
    display: grid;
    grid-template-columns: repeat(auto-fit, minmax(250px, 1fr));
    gap: 20px;
    margin-top: 20px;
}

.speaker-slot {
    display: flex;
    align-items: center;
    gap: 16px;
    padding: 16px;
    background: var(--gray-50);
    border-radius: var(--radius);
    border: 1px solid var(--gray-200);
}

.speaker-time {
    font-size: 0.875rem;
    font-weight: 600;
    color: var(--primary);
    min-width: 80px;
}

.speaker-avatar {
    width: 50px;
    height: 50px;
    background: var(--gray-300);
    border-radius: 50%;
    display: flex;
    align-items: center;
    justify-content: center;
    font-weight: 600;
    color: var(--gray-600);
    font-size: 0.75rem;
    flex-shrink: 0;
}

.speaker-info {
    display: flex;
    flex-direction: column;
}

.speaker-name {
    font-weight: 600;
    color: var(--gray-800);
}

.speaker-affiliation {
    font-size: 0.875rem;
    color: var(--gray-500);
}

/* Speakers List */
.speakers {
    background: var(--gray-50);
}

.speakers-intro {
    text-align: center;
    max-width: 700px;
    margin: -30px auto 50px;
    color: var(--gray-600);
    font-size: 1.1rem;
}

.speakers-list {
    display: grid;
    grid-template-columns: repeat(auto-fill, minmax(280px, 1fr));
    gap: 24px;
}

.speaker-card {
    background: var(--white);
    border-radius: var(--radius-lg);
    padding: 24px;
    display: flex;
    align-items: center;
    gap: 16px;
    box-shadow: var(--shadow);
    transition: all 0.3s;
    position: relative;
}

.speaker-card:hover {
    transform: translateY(-4px);
    box-shadow: var(--shadow-lg);
}

.speaker-photo {
    width: 70px;
    height: 70px;
    border-radius: 50%;
    object-fit: cover;
    flex-shrink: 0;
}

.speaker-photo.speaker-initials {
    display: flex;
    align-items: center;
    justify-content: center;
    font-size: 1.25rem;
    font-weight: 600;
    color: var(--white);
}

.speaker-details {
    flex: 1;
    min-width: 0;
}

.speaker-details h3 {
    font-size: 1.1rem;
    font-weight: 600;
    color: var(--primary);
    margin-bottom: 4px;
}

.speaker-title {
    font-size: 0.85rem;
    color: var(--accent);
    font-weight: 500;
    margin-bottom: 2px;
}

.speaker-card .speaker-affiliation {
    font-size: 0.85rem;
    color: var(--gray-500);
    line-height: 1.3;
}

.speaker-link {
    position: absolute;
    top: 12px;
    right: 12px;
    font-size: 0.75rem;
    color: var(--primary);
    text-decoration: none;
    opacity: 0;
    transition: opacity 0.2s;
}

.speaker-card:hover .speaker-link {
    opacity: 1;
}

.speaker-link:hover {
    text-decoration: underline;
}

/* Organizers */
.organizers-grid {
    display: grid;
    grid-template-columns: repeat(5, 1fr);
    gap: 24px;
    max-width: 1100px;
    margin: 0 auto;
}

.organizer-card {
    text-align: center;
    padding: 32px 24px;
    background: var(--white);
    border-radius: var(--radius-lg);
    box-shadow: var(--shadow);
    transition: all 0.3s;
}

.organizer-card:hover {
    transform: translateY(-4px);
    box-shadow: var(--shadow-lg);
}

.organizer-avatar {
    width: 100px;
    height: 100px;
    border-radius: 50%;
    display: flex;
    align-items: center;
    justify-content: center;
    font-size: 2rem;
    font-weight: 600;
    color: var(--white);
    margin: 0 auto 20px;
}

.organizer-photo {
    width: 100px;
    height: 100px;
    border-radius: 50%;
    object-fit: cover;
    margin: 0 auto 20px;
}

.organizer-card h3 {
    font-size: 1.15rem;
    font-weight: 600;
    color: var(--primary);
    margin-bottom: 8px;
}

.organizer-affiliation {
    font-size: 0.9rem;
    color: var(--gray-500);
    line-height: 1.5;
    margin-bottom: 16px;
}

.organizer-link {
    color: var(--accent);
    text-decoration: none;
    font-weight: 500;
    font-size: 0.9rem;
    transition: color 0.2s;
}

.organizer-link:hover {
    color: var(--accent-dark);
}

/* Venue */
.venue {
    background: var(--gray-50);
}

.venue-grid {
    display: grid;
    grid-template-columns: repeat(auto-fit, minmax(400px, 1fr));
    gap: 32px;
}

.venue-card {
    background: var(--white);
    border-radius: var(--radius-lg);
    overflow: hidden;
    box-shadow: var(--shadow);
}

.venue-image {
    height: 200px;
    background-size: cover;
    background-position: center;
}

.venue-image.flatiron {
    background: linear-gradient(135deg, var(--primary) 0%, var(--primary-light) 100%);
    display: flex;
    align-items: center;
    justify-content: center;
}

.venue-image.flatiron::after {
    content: 'Flatiron Institute';
    color: var(--white);
    font-family: var(--font-heading);
    font-size: 1.5rem;
    font-weight: 600;
}

.venue-image.ibm {
    background: linear-gradient(135deg, var(--ibm-blue) 0%, #0043ce 100%);
    display: flex;
    align-items: center;
    justify-content: center;
}

.venue-image.ibm::after {
    content: 'IBM One Madison';
    color: var(--white);
    font-family: var(--font-heading);
    font-size: 1.5rem;
    font-weight: 600;
}

.venue-info {
    padding: 28px;
}

.venue-days {
    display: inline-block;
    background: var(--accent);
    color: var(--white);
    padding: 4px 12px;
    border-radius: 20px;
    font-size: 0.8rem;
    font-weight: 600;
    margin-bottom: 12px;
}

.venue-info h3 {
    font-size: 1.35rem;
    font-weight: 600;
    color: var(--gray-800);
    margin-bottom: 8px;
}

.venue-address {
    display: block;
    color: var(--gray-600);
    font-size: 0.95rem;
    margin-bottom: 16px;
    text-decoration: none;
}

.venue-address:hover {
    text-decoration: underline;
}

.venue-description {
    color: var(--gray-500);
    font-size: 0.9rem;
    line-height: 1.6;
}

/* Register */
.register-content {
    display: grid;
    grid-template-columns: 1fr 1.2fr;
    gap: 48px;
    align-items: start;
}

.register-info h3 {
    font-size: 1.5rem;
    font-weight: 600;
    color: var(--gray-800);
    margin-bottom: 16px;
}

.register-info > p {
    color: var(--gray-600);
    font-size: 1.05rem;
    margin-bottom: 32px;
}

.register-details {
    display: flex;
    flex-direction: column;
    gap: 24px;
}

.register-item h4 {
    font-size: 1.1rem;
    font-weight: 600;
    color: var(--primary);
    margin-bottom: 12px;
}

.register-item ul {
    list-style: none;
    padding-left: 0;
}

.register-item li {
    position: relative;
    padding-left: 20px;
    color: var(--gray-600);
    margin-bottom: 8px;
}

.register-item li::before {
    content: '→';
    position: absolute;
    left: 0;
    color: var(--accent);
}

.register-item p {
    color: var(--gray-600);
}

.register-form {
    background: var(--gray-50);
    border-radius: var(--radius-lg);
    overflow: hidden;
    min-height: 500px;
}

.form-placeholder {
    display: flex;
    flex-direction: column;
    align-items: center;
    justify-content: center;
    padding: 60px 40px;
    text-align: center;
    height: 100%;
    min-height: 500px;
}

.form-placeholder h4 {
    font-size: 1.5rem;
    font-weight: 600;
    color: var(--gray-800);
    margin-bottom: 16px;
}

.form-placeholder p {
    color: var(--gray-500);
    max-width: 400px;
}

.form-placeholder .form-note {
    margin-top: 24px;
    padding: 16px 24px;
    background: var(--white);
    border-radius: var(--radius);
    border: 1px dashed var(--gray-300);
    font-size: 0.9rem;
    color: var(--gray-500);
}

.register-form iframe {
    width: 100%;
    min-height: 800px;
    border: none;
}

.form-cta {
    display: flex;
    flex-direction: column;
    align-items: center;
    justify-content: center;
    padding: 60px 40px;
    text-align: center;
    height: 100%;
    min-height: 400px;
}

.form-cta h4 {
    font-size: 1.75rem;
    font-weight: 600;
    color: var(--gray-800);
    margin-bottom: 12px;
}

.form-cta > p {
    color: var(--gray-500);
    margin-bottom: 32px;
    font-size: 1.05rem;
}

.apply-btn {
    display: inline-flex;
    align-items: center;
    gap: 10px;
    background: var(--accent);
    color: var(--white);
    padding: 18px 36px;
    border-radius: var(--radius);
    font-weight: 600;
    font-size: 1.1rem;
    text-decoration: none;
    transition: all 0.2s;
    box-shadow: var(--shadow-lg);
}

.apply-btn:hover {
    background: var(--accent-dark);
    transform: translateY(-2px);
    box-shadow: var(--shadow-xl);
}

.deadline-note {
    margin-top: 24px;
    color: var(--gray-600);
    font-size: 0.95rem;
}

/* Footer */
.footer {
    background: var(--primary-dark);
    color: var(--white);
    padding: 60px 0;
}

.footer-content {
    text-align: center;
}

.footer-logos {
    display: flex;
    justify-content: center;
    gap: 40px;
    margin-bottom: 32px;
}

.footer-logo {
    height: 32px;
    filter: brightness(0) invert(1);
    opacity: 0.8;
}

.footer-text {
    font-size: 1.1rem;
    opacity: 0.9;
    margin-bottom: 16px;
}

.footer-contact {
    font-size: 0.95rem;
    opacity: 0.7;
}

.footer-contact a {
    color: var(--accent-light);
    text-decoration: none;
}

.footer-contact a:hover {
    text-decoration: underline;
}

/* Mobile Responsive */
@media (max-width: 768px) {
    .nav-menu {
        position: fixed;
        top: 72px;
        left: 0;
        right: 0;
        background: var(--white);
        flex-direction: column;
        padding: 24px;
        gap: 16px;
        border-bottom: 1px solid var(--gray-200);
        display: none;
    }

    .nav-menu.active {
        display: flex;
    }

    .nav-toggle {
        display: flex;
    }

    .hero {
        padding: 100px 20px 60px;
    }

    .hero-logos img {
        height: 28px;
    }

    .hero-details {
        flex-direction: column;
        gap: 16px;
    }

    .section {
        padding: 60px 0;
    }

    .schedule-tabs {
        flex-direction: column;
    }

    .tab-btn {
        width: 100%;
    }

    .tab-content {
        padding: 24px;
    }

    .speakers-grid {
        grid-template-columns: 1fr;
    }

    .venue-grid {
        grid-template-columns: 1fr;
    }

    .register-content {
        grid-template-columns: 1fr;
    }

    .organizers-grid {
        grid-template-columns: repeat(2, 1fr);
        gap: 20px;
    }

    .organizer-card {
        padding: 24px 16px;
    }

    .organizer-avatar {
        width: 80px;
        height: 80px;
        font-size: 1.5rem;
    }
}

@media (max-width: 480px) {
    .organizers-grid {
        grid-template-columns: 1fr;
    }
}