:root {
  --bg: #252525;           /* page background                         */

  --title-color: #e1e1e1;  /* 1. the "Yitian Sun" title               */
  --nav-color:   #ededed;  /* 2. the "About / Research" tabs          */
  --rule-color:  #6a6a6a;  /* 3. the two bars + copyright line        */
  --text-color:  #c9c9c9;  /* 4. body text                            */

  --link-color:  #ff7878;  /* in-text links                           */
  --accent:      #ff7878;  /* accent color                            */

  --content-width: 1080px;
}

* { box-sizing: border-box; }

html, body { margin: 0; }

body {
  background: var(--bg);
  color: var(--text-color);
  font-family: -apple-system, BlinkMacSystemFont, "Segoe UI", Roboto,
               Helvetica, Arial, sans-serif;
  font-size: 18px;
  line-height: 1.55;
  -webkit-font-smoothing: antialiased;
}

/* Centered, generously-margined content block */
.container {
  max-width: var(--content-width);
  margin: 0 auto;
  padding: 64px 48px;
}

/* ---- Header: title left, tabs (with underline) right ---- */
header {
  display: flex;
  justify-content: space-between;
  align-items: baseline;
  gap: 2rem;
  flex-wrap: wrap;
}

.site-title {
  margin: 0;
  font-size: 3rem;
  font-weight: 400;
  letter-spacing: 0.5px;
  color: var(--title-color);
}

nav {
  display: flex;
  gap: 1.75rem;
  padding-bottom: 0.6rem;
  border-bottom: 1px solid var(--rule-color);   /* bar #1 */
}
nav a {
  color: var(--nav-color);
  text-decoration: none;
  font-size: 1.2rem;
}
nav a:hover { color: var(--accent); }

/* ---- Main ---- */
main { margin-top: 4rem; }

/* Home intro: bio left, portrait right */
.intro {
  display: grid;
  grid-template-columns: minmax(0, 1fr) auto;
  gap: 4rem;
  align-items: start;
}
.bio { max-width: 30rem; }
.bio p { margin: 0 0 1.4rem; }

.portrait {
  width: 300px;
  height: 300px;
  border-radius: 28px;
  background: var(--accent);
}
img.portrait { object-fit: cover; }

a { color: var(--link-color); }

/* Research page bits */
h2 { color: var(--text-color); font-weight: 500; }

/* Research entries: square gif left, text right */
.research-list {
  display: flex;
  flex-direction: column;
  gap: 3.5rem;
  margin-top: 3rem;
}

.research-item {
  display: grid;
  grid-template-columns: 280px minmax(0, 1fr);
  gap: 2.5rem;
  align-items: start;
}

.research-gif {
  display: block;
  width: 280px;
  border-radius: 16px;
  overflow: hidden;
  background: #1c1c1c;
}
.research-gif img {
  width: 100%;
  height: auto;
  display: block;
}

.research-body h2 {
  margin: 0 0 0.6rem;
  font-size: 1.4rem;
  line-height: 1.3;
}
.research-body p { margin: 0 0 0.9rem; }

.research-links { font-size: 0.95rem; color: var(--rule-color); }
.research-links a { margin: 0 0.1rem; }

.research-more { margin-top: 3.5rem; }

/* ---- Footer: full-width bar + centered copyright ---- */
footer {
  margin-top: 4rem;
  border-top: 1px solid var(--rule-color);       /* bar #2 */
  padding-top: 1.5rem;
  text-align: center;
}
footer p {
  margin: 0;
  color: var(--rule-color);
  font-size: 0.95rem;
}

/* ---- Narrow screens: stack ---- */
@media (max-width: 720px) {
  .container { padding: 40px 24px; }
  .intro { grid-template-columns: 1fr; gap: 2.5rem; }
  .portrait { width: 100%; height: auto; aspect-ratio: 1 / 1; }
  header { gap: 1rem; }
  .site-title { font-size: 2.25rem; }

  .research-item { grid-template-columns: 1fr; gap: 1.5rem; }
  .research-gif { width: 100%; max-width: 360px; }
}