/* Colors */
:root {
    --primary-blue: #1a3a5c;
    --primary-blue-light: #2d5a87;
    --columbia-blue: #1a3a5c;
    --columbia-navy: #1a3a5c;
    --columbia-dark: #0f2840;
    --white: #ffffff;
    --gray-50: #f9fafb;
    --gray-100: #f3f4f6;
    --gray-200: #e5e7eb;
    --gray-300: #d1d5db;
    --gray-600: #4b5563;
    --gray-700: #374151;
    --gray-800: #1f2937;
    --gray-900: #111827;
}

/* Reset and Base */
*, *::before, *::after {
    box-sizing: border-box;
}

html {
    scroll-behavior: smooth;
}

body {
    margin: 0;
    font-family: 'Source Sans 3', -apple-system, BlinkMacSystemFont, 'Segoe UI', sans-serif;
    font-size: 16px;
    line-height: 1.6;
    color: var(--gray-800);
    background: var(--white);
}

.container {
    max-width: 900px;
    margin: 0 auto;
    padding: 0 24px;
}

/* Typography */
h1, h2, h3 {
    font-family: 'Source Serif 4', Georgia, serif;
    font-weight: 600;
    line-height: 1.2;
}

h1 {
    font-size: 2.25rem;
    margin: 0;
}

h2 {
    font-size: 1.75rem;
    color: var(--columbia-navy);
    margin: 0 0 1.5rem 0;
    padding-bottom: 0.5rem;
    border-bottom: 2px solid var(--columbia-blue);
}

h3 {
    font-size: 1.25rem;
    color: var(--gray-700);
    margin: 2rem 0 1rem 0;
}

p {
    margin: 0 0 1rem 0;
}

a {
    color: var(--columbia-navy);
    text-decoration: none;
}

a:hover {
    text-decoration: underline;
}

/* Header */
header {
    background: var(--white);
    border-bottom: 1px solid var(--gray-200);
    padding: 1rem 0;
}

header .container {
    display: flex;
    align-items: center;
    gap: 2rem;
}

header img {
    height: 40px;
    margin: 0;
}

header h1 {
    color: #1a3a5c;
    margin: 0;
    font-size: 1.25rem;
    font-weight: 600;
}

.subtitle {
    font-size: 1.125rem;
    color: var(--columbia-blue);
    margin: 0;
    font-weight: 300;
    letter-spacing: 0.5px;
}

/* Navigation */
nav {
    background: var(--white);
    border-bottom: 1px solid var(--gray-200);
    position: sticky;
    top: 0;
    z-index: 100;
    margin-bottom: 2rem;
}

nav .container {
    display: flex;
    gap: 2rem;
    padding-top: 1rem;
    padding-bottom: 1rem;
}

nav a {
    color: var(--gray-600);
    font-weight: 500;
    font-size: 0.9375rem;
    transition: color 0.2s;
}

nav a:hover {
    color: var(--columbia-navy);
    text-decoration: none;
}

nav a.nav-active {
    color: var(--columbia-navy);
    font-weight: 600;
}

/* Main Content */
main {
    padding-bottom: 4rem;
}

section {
    padding: 3rem 0;
}

section#about {
    background: var(--white);
    border-bottom: 1px solid var(--gray-200);
}

section#about p {
    font-size: 1.0625rem;
    color: var(--gray-700);
    max-width: 720px;
}

/* Badge */
.badge {
    display: inline-block;
    background: var(--columbia-blue);
    color: var(--white);
    font-size: 0.75rem;
    font-weight: 600;
    padding: 0.25rem 0.75rem;
    border-radius: 100px;
    vertical-align: middle;
    margin-left: 0.75rem;
    font-family: 'Source Sans 3', sans-serif;
    text-transform: uppercase;
    letter-spacing: 0.5px;
}

/* Event Details */
.event-details {
    display: flex;
    gap: 3rem;
    margin-bottom: 2rem;
    padding: 1.5rem;
    background: var(--gray-50);
    border-radius: 8px;
    border: 1px solid var(--gray-200);
}

.detail strong {
    display: block;
    font-size: 0.75rem;
    text-transform: uppercase;
    letter-spacing: 0.5px;
    color: var(--gray-600);
    margin-bottom: 0.25rem;
}

.detail span {
    color: var(--gray-800);
    font-size: 0.9375rem;
    line-height: 1.5;
}

/* Schedule */
.schedule {
    border: 1px solid var(--gray-200);
    border-radius: 8px;
    overflow: hidden;
}

.session-header {
    background: var(--columbia-navy);
    color: var(--white);
    padding: 0.5rem 1.25rem;
    font-weight: 600;
    font-size: 0.875rem;
    text-transform: uppercase;
    letter-spacing: 0.5px;
}

.time-block {
    display: flex;
    padding: 1rem 1.25rem;
    border-bottom: 1px solid var(--gray-100);
    gap: 1.5rem;
}

.time-block:last-child {
    border-bottom: none;
}

.time-block.break {
    background: var(--gray-50);
}

.time-block.break .event {
    color: var(--gray-600);
    font-style: italic;
}

.time {
    flex-shrink: 0;
    width: 100px;
    font-size: 0.875rem;
    color: var(--gray-600);
    font-weight: 500;
}

.event {
    flex-grow: 1;
}

.presenter {
    font-weight: 600;
    color: var(--gray-900);
}

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

.paper-title {
    color: var(--gray-700);
    font-size: 0.9375rem;
    margin-top: 0.25rem;
}

/* Compact schedule for past years */
.schedule.compact .time-block {
    padding: 0.75rem 1.25rem;
}

.schedule.compact .session-header {
    padding: 0.375rem 1.25rem;
    font-size: 0.8125rem;
}

/* Year Section */
.year-section {
    background: var(--white);
}

/* Accordions for past years */
.year-accordion {
    border: 1px solid var(--gray-200);
    border-radius: 8px;
    margin-bottom: 1rem;
    overflow: hidden;
}

.year-accordion summary {
    display: flex;
    justify-content: space-between;
    align-items: center;
    padding: 1rem 1.25rem;
    cursor: pointer;
    background: var(--white);
    transition: background 0.2s;
    list-style: none;
}

.year-accordion summary::-webkit-details-marker {
    display: none;
}

.year-accordion summary::before {
    content: '+';
    font-size: 1.25rem;
    font-weight: 300;
    color: var(--gray-400);
    margin-right: 1rem;
    transition: transform 0.2s;
}

.year-accordion[open] summary::before {
    content: '-';
}

.year-accordion summary:hover {
    background: var(--gray-50);
}

.year-title {
    font-family: 'Source Serif 4', Georgia, serif;
    font-weight: 600;
    font-size: 1.125rem;
    color: var(--columbia-navy);
}

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

.accordion-content {
    padding: 0 1.25rem 1.25rem 1.25rem;
    border-top: 1px solid var(--gray-100);
}

.accordion-content .location {
    color: var(--gray-600);
    font-size: 0.9375rem;
    margin: 1rem 0;
}

.accordion-content .note {
    color: var(--gray-500);
    font-style: italic;
    font-size: 0.875rem;
}

.accordion-content .schedule {
    margin-top: 1rem;
}

/* Organizers */
.organizers-section {
    background: var(--columbia-navy);
    color: var(--white);
}

.organizers-section h2 {
    color: var(--white);
    border-bottom-color: rgba(255, 255, 255, 0.2);
}

.organizers-intro {
    color: var(--columbia-blue);
    margin-bottom: 2rem;
}

.organizers-grid {
    display: grid;
    grid-template-columns: repeat(auto-fill, minmax(180px, 1fr));
    gap: 1rem;
}

.organizer {
    background: rgba(255, 255, 255, 0.1);
    padding: 0.75rem 1rem;
    border-radius: 6px;
    font-weight: 500;
}

.contact {
    margin-top: 2rem;
    color: var(--columbia-blue);
    font-size: 0.9375rem;
}

/* Organizers Light Theme */
.organizers-grid-light {
    display: grid;
    grid-template-columns: repeat(auto-fill, minmax(180px, 1fr));
    gap: 1rem;
    margin-top: 1.5rem;
}

.organizer-light {
    background: var(--gray-100);
    padding: 0.75rem 1rem;
    border-radius: 6px;
    font-weight: 500;
    color: var(--gray-800);
}

/* Footer */
footer {
    background: var(--white);
    border-top: 1px solid var(--gray-200);
    color: var(--gray-500);
    padding: 1.5rem 0;
    text-align: center;
    font-size: 0.8125rem;
}

footer p {
    margin: 0.25rem 0;
}

/* Featured Card */
.featured-card {
    display: block;
    background: var(--gray-50);
    border: 2px solid var(--columbia-blue);
    border-radius: 8px;
    padding: 2rem;
    margin-top: 2rem;
    text-decoration: none;
    transition: box-shadow 0.2s, transform 0.2s;
}

.featured-card:hover {
    text-decoration: none;
    box-shadow: 0 4px 12px rgba(0, 43, 127, 0.15);
    transform: translateY(-2px);
}

.featured-card h3 {
    font-family: 'Source Serif 4', Georgia, serif;
    font-size: 1.5rem;
    color: var(--columbia-navy);
    margin: 0.5rem 0 0.25rem 0;
}

.featured-card p {
    color: var(--gray-600);
    margin: 0;
}

.featured-card .featured-date {
    margin-top: 0.75rem;
    font-size: 0.9375rem;
}

.featured-badge {
    display: inline-block;
    background: var(--columbia-navy);
    color: var(--white);
    font-size: 0.75rem;
    font-weight: 600;
    padding: 0.25rem 0.75rem;
    border-radius: 100px;
    text-transform: uppercase;
    letter-spacing: 0.5px;
}

/* Mobile Responsiveness */
@media (max-width: 640px) {
    h1 {
        font-size: 1.75rem;
    }

    h2 {
        font-size: 1.5rem;
    }

    header {
        padding: 3rem 0;
    }

    nav .container {
        gap: 1rem;
        flex-wrap: wrap;
        justify-content: center;
    }

    .event-details {
        flex-direction: column;
        gap: 1.5rem;
    }

    .time-block {
        flex-direction: column;
        gap: 0.5rem;
    }

    .time {
        width: auto;
    }

    .badge {
        display: block;
        margin: 0.5rem 0 0 0;
    }

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