/* ===== CSS Variables / Theming ===== */
:root {
  --bg: #ffffff;
  --bg-secondary: #f0f3f8;
  --text: #1a1a1a;
  --text-secondary: #555;
  --accent: #011F5B; /* Penn blue */
  --accent-light: #82afd3;
  --accent-subtle: rgba(1, 31, 91, 0.06);
  --border: #e0e0e0;
  --card-bg: #ffffff;
  --card-shadow: 0 1px 4px rgba(0,0,0,0.06);
  --hover-bg: #f0f4f8;
  --tag-bg: #eef2f7;
  --tag-text: #011F5B;
  --link: #990000; /* Penn red */
  --nav-bg: rgba(255,255,255,0.92);
  --toggle-bg: #e0e0e0;
  --hero-bg: linear-gradient(135deg, #f0f3f8 0%, #e8edf5 100%);
  --photo-shadow: 0 4px 20px rgba(1, 31, 91, 0.15);
}

[data-theme="dark"] {
  --bg: #111318;
  --bg-secondary: #1a1d24;
  --text: #e0e0e0;
  --text-secondary: #999;
  --accent: #82afd3;
  --accent-light: #011F5B;
  --accent-subtle: rgba(130, 175, 211, 0.08);
  --border: #2a2d35;
  --card-bg: #1a1d24;
  --card-shadow: 0 1px 4px rgba(0,0,0,0.3);
  --hover-bg: #22252d;
  --tag-bg: #22252d;
  --tag-text: #82afd3;
  --link: #e07070;
  --nav-bg: rgba(17, 19, 24, 0.92);
  --toggle-bg: #333;
  --hero-bg: linear-gradient(135deg, #1a1d24 0%, #1e2230 100%);
  --photo-shadow: 0 4px 20px rgba(0, 0, 0, 0.4);
}

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

html { scroll-behavior: smooth; }

body {
  font-family: 'Source Sans 3', 'Source Sans Pro', -apple-system, BlinkMacSystemFont, sans-serif;
  background: var(--bg);
  color: var(--text);
  line-height: 1.65;
  font-size: 16px;
  transition: background 0.3s, color 0.3s;
}

a {
  color: var(--link);
  text-decoration: none;
  transition: opacity 0.2s;
}
a:hover { opacity: 0.75; }

/* ===== Navigation ===== */
.navbar {
  position: sticky;
  top: 0;
  z-index: 100;
  background: var(--nav-bg);
  box-shadow: 0 1px 8px rgba(0,0,0,0.08);
  backdrop-filter: blur(12px);
  -webkit-backdrop-filter: blur(12px);
  transition: background 0.3s;
}

.nav-inner {
  max-width: 860px;
  margin: 0 auto;
  padding: 0.75rem 1.5rem;
  display: flex;
  justify-content: space-between;
  align-items: center;
}

.nav-name {
  font-weight: 700;
  font-size: 1.1rem;
  color: var(--accent);
  text-decoration: none;
  display: flex;
  align-items: center;
  gap: 0.5rem;
}
.nav-name:hover { opacity: 0.75; }

.nav-logo {
  height: 28px;
  width: auto;
}

.nav-links {
  display: flex;
  align-items: center;
  gap: 1.8rem;
  list-style: none;
}

.nav-links a {
  color: var(--text-secondary);
  font-size: 0.92rem;
  font-weight: 500;
  text-decoration: none;
  transition: color 0.2s;
}
.nav-links a:hover,
.nav-links a.active {
  color: var(--accent);
}
.nav-links a.active {
  border-bottom: 2px solid var(--accent);
  padding-bottom: 2px;
}

/* Theme toggle */
.theme-toggle {
  background: none;
  border: none;
  cursor: pointer;
  font-size: 1.15rem;
  color: var(--text-secondary);
  padding: 0.25rem;
  display: flex;
  align-items: center;
  transition: color 0.2s;
}
.theme-toggle:hover { color: var(--accent); }

/* Mobile nav */
.nav-hamburger {
  display: none;
  background: none;
  border: none;
  cursor: pointer;
  color: var(--text);
  font-size: 1.4rem;
  padding: 0.25rem;
}

@media (max-width: 640px) {
  .nav-hamburger { display: block; }
  .nav-links {
    display: none;
    position: absolute;
    top: 100%;
    left: 0;
    right: 0;
    background: var(--nav-bg);
    border-bottom: 1px solid var(--border);
    flex-direction: column;
    padding: 1rem 1.5rem;
    gap: 1rem;
  }
  .nav-links.open { display: flex; }
}

/* ===== Layout ===== */
.container {
  max-width: 860px;
  margin: 0 auto;
  padding: 0 1.5rem 4rem;
}

/* ===== Hero / About Section ===== */
#about {
  background: var(--hero-bg);
  margin: 0 -1.5rem;
  padding: 3rem 1.5rem 2.5rem;
  border-radius: 0 0 12px 12px;
  margin-bottom: 2rem;
  position: relative;
  overflow: hidden;
}

#particles {
  position: absolute;
  inset: 0;
  width: 100%;
  height: 100%;
  pointer-events: none;
}

.home-hero {
  display: grid;
  grid-template-columns: 1fr 220px;
  gap: 2.5rem;
  align-items: start;
  max-width: 860px;
  margin: 0 auto;
}

.home-hero h1 {
  font-family: Georgia, 'Times New Roman', serif;
  font-size: 2.2rem;
  font-weight: 700;
  color: var(--accent);
  margin-bottom: 1rem;
  letter-spacing: -0.01em;
}

/* Typewriter */
.typewriter::after {
  content: '|';
  animation: blink-cursor 0.7s step-end infinite;
  color: var(--accent);
}
.typewriter-done::after {
  animation: blink-cursor 0.7s step-end 3;
  animation-fill-mode: forwards;
}
.typewriter-done::after { opacity: 0; animation: blink-cursor 0.7s step-end 3 forwards; }

@keyframes blink-cursor {
  0%, 100% { opacity: 1; }
  50% { opacity: 0; }
}

/* Gradient name */
.home-hero h1 {
  background: linear-gradient(135deg, var(--accent) 0%, var(--link) 100%);
  -webkit-background-clip: text;
  -webkit-text-fill-color: transparent;
  background-clip: text;
}

.home-bio {
  font-size: 1.02rem;
  color: var(--text);
  line-height: 1.7;
}

.home-photo {
  width: 220px;
  height: 280px;
  object-fit: cover;
  object-position: center top;
  border-radius: 8px;
  box-shadow: var(--photo-shadow);
}

.contact-block {
  margin-top: 1.5rem;
  padding-top: 1rem;
  border-top: 1px solid var(--border);
  font-size: 0.92rem;
  color: var(--text-secondary);
  line-height: 1.8;
}

@media (max-width: 640px) {
  #about {
    padding: 2rem 1.5rem;
  }
  .home-hero {
    grid-template-columns: 1fr;
    text-align: center;
  }
  .home-photo {
    margin: 0 auto;
    width: 180px;
    height: 230px;
    order: -1;
  }
}

/* ===== Section Titles ===== */
section#research,
section#teaching {
  padding-top: 2rem;
  margin-bottom: 3rem;
}

.section-title {
  font-family: Georgia, 'Times New Roman', serif;
  font-size: 1.7rem;
  font-weight: 700;
  color: var(--accent);
  margin-bottom: 1.5rem;
  padding-bottom: 0.5rem;
  border-bottom: 2px solid transparent;
  border-image: linear-gradient(to right, var(--accent), var(--link), transparent) 1;
  letter-spacing: -0.01em;
}

.subsection-title {
  font-family: Georgia, 'Times New Roman', serif;
  font-size: 1.2rem;
  font-weight: 600;
  color: var(--accent);
  margin: 2.5rem 0 1rem;
  letter-spacing: -0.01em;
}

/* ===== Paper Cards ===== */
.paper {
  background: var(--card-bg);
  border-left: 3px solid transparent;
  border-image: linear-gradient(to bottom, var(--accent), var(--link)) 1;
  border-bottom: none;
  border-radius: 4px;
  padding: 1rem 1.2rem;
  margin-bottom: 0.75rem;
  box-shadow: var(--card-shadow);
  transition: box-shadow 0.2s, transform 0.2s;
}
.paper:hover {
  box-shadow: 0 2px 16px rgba(1, 31, 91, 0.15), 0 0 8px rgba(1, 31, 91, 0.08);
}
[data-theme="dark"] .paper:hover {
  box-shadow: 0 2px 16px rgba(130, 175, 211, 0.15), 0 0 8px rgba(130, 175, 211, 0.08);
}

.paper-header {
  display: flex;
  justify-content: space-between;
  align-items: flex-start;
  gap: 1rem;
  cursor: pointer;
}

.paper-header:hover .paper-title {
  color: var(--accent);
}

.paper-title {
  font-weight: 600;
  font-size: 1.05rem;
  color: var(--text);
  transition: color 0.2s;
}

.paper-meta {
  font-size: 0.9rem;
  color: var(--text-secondary);
  margin-top: 0.25rem;
  line-height: 1.5;
}

.paper-meta em {
  font-style: italic;
  color: var(--accent);
  font-weight: 600;
}

.paper-links {
  margin-top: 0.5rem;
  display: flex;
  flex-wrap: wrap;
  gap: 0.5rem;
}

.paper-links a {
  font-size: 0.8rem;
  font-weight: 600;
  color: var(--link);
  padding: 0.2rem 0.55rem;
  border: 1px solid var(--link);
  border-radius: 3px;
  transition: background 0.2s, color 0.2s;
}
.paper-links a:hover {
  background: var(--link);
  color: #fff;
  opacity: 1;
}

.paper-chevron {
  flex-shrink: 0;
  width: 20px;
  height: 20px;
  margin-top: 0.2rem;
  color: var(--text-secondary);
  transition: transform 0.3s;
}
.paper.open .paper-chevron {
  transform: rotate(180deg);
}

.paper-abstract {
  max-height: 0;
  overflow: hidden;
  transition: max-height 0.4s ease, padding 0.3s ease;
  font-size: 0.9rem;
  color: var(--text-secondary);
  line-height: 1.7;
}
.paper.open .paper-abstract {
  max-height: 600px;
  padding-top: 0.75rem;
}

.paper-note {
  font-size: 0.85rem;
  color: var(--text-secondary);
  font-style: italic;
  margin-top: 0.3rem;
}

/* ===== Teaching ===== */
.teaching-group {
  margin-bottom: 2rem;
}

.teaching-group h2 {
  font-family: Georgia, 'Times New Roman', serif;
  font-size: 1.1rem;
  font-weight: 600;
  color: var(--accent);
  margin-bottom: 0.75rem;
  padding-bottom: 0.3rem;
  border-bottom: 1px solid var(--border);
}

.teaching-group h3 {
  font-size: 0.95rem;
  font-weight: 600;
  color: var(--text);
  margin: 1rem 0 0.4rem;
}

.teaching-item {
  padding: 0.35rem 0;
  font-size: 0.93rem;
  color: var(--text-secondary);
}

.teaching-item strong {
  color: var(--text);
}

/* ===== Easter egg: econ rain ===== */
.econ-rain-overlay {
  position: fixed;
  inset: 0;
  z-index: 9999;
  pointer-events: none;
  overflow: hidden;
}

.econ-rain-drop {
  position: absolute;
  top: -2rem;
  color: var(--accent);
  font-family: 'Source Sans 3', monospace;
  font-weight: 600;
  white-space: nowrap;
  animation: econ-fall linear forwards;
}

@keyframes econ-fall {
  to { top: 105vh; }
}

/* ===== Footer ===== */
.footer {
  text-align: center;
  padding: 2rem 1.5rem;
  font-size: 0.82rem;
  color: var(--text-secondary);
  border-top: 1px solid var(--border);
}