/* ================================
   Global Variables (Light Mode Default)
=================================== */
:root {
  --bg: #f3f6fb;
  --bg-2: #eaf1fb;
  --card: #ffffff;
  --card-hover: #f7fbff;
  --border: #dde6f7;
  --text: #0e1a2b;
  --muted: #4f5b78;
  --accent: #0069ff;
  --accent-2: #6a44ff;
  --success: #148f5f;
  --shadow: 0 12px 28px rgba(16,46,84,0.15);
}

* { box-sizing: border-box; }
html, body { height: 100%; }
body {
  margin: 0;
  font: 18px/1.7 "Inter", "Segoe UI", Roboto, Arial, sans-serif; /* larger base font */
  color: var(--text);
  background: radial-gradient(1200px 800px at 10% 10%, #ffffff, var(--bg));
}

/* ================================
   Top Navigation Bar
=================================== */
.topbar {
  position: sticky;
  top: 0;
  z-index: 50;
  display: flex;
  align-items: center;
  gap: 1rem;
  padding: 1rem 1.25rem; /* more padding */
  background: rgba(255,255,255,0.9);
  backdrop-filter: saturate(150%) blur(12px);
  border-bottom: 1px solid var(--border);
}

.brand {
  display: flex;
  align-items: center;
  gap: 0.6rem;
  padding-right: 1rem;
  border-right: 1px dashed var(--border);
}
.brand-mark { font-size: 1.4rem; }
.brand-name { font-weight: 600; letter-spacing: 0.3px; color: var(--muted); font-size: 1.1rem; }

.nav-links {
  display: flex;
  flex-wrap: wrap;
  gap: 0.6rem 1rem;
}
.nav-links a {
  color: var(--text);
  text-decoration: none;
  padding: 0.55rem 0.9rem;
  border-radius: 999px;
  border: 1px solid transparent;
  transition: all .2s ease;
  background: rgba(0,0,0,0.04);
  font-size: 1rem;
}
.nav-links a:hover,
.nav-links a:focus-visible {
  border-color: var(--accent);
  box-shadow: 0 0 0 4px rgba(0,105,255,0.15);
  outline: none;
}

/* ================================
   Subtopic Navigation Cards
=================================== */
.subnav {
  display: flex;
  flex-wrap: wrap;
  gap: 1.2rem;
  margin: 1.5rem 0 2.5rem 0;
}
.subnav a {
  flex: 1 1 180px;
  text-decoration: none;
}
.subnav .nav-card {
  background: #e6f2f5;
  border-radius: 8px;
  padding: 1rem 1.2rem;
  text-align: center;
  font-size: 1.05rem;
  color: #004466;
  box-shadow: 0 3px 8px rgba(0,0,0,0.08);
  transition: background 0.2s ease, transform 0.2s ease;
}
.subnav .nav-card:hover {
  background: #cce6eb;
  transform: translateY(-3px);
}

/* ================================
   Content Container
=================================== */
.content {
  max-width: 1200px;
  margin: 1.5rem auto 3rem;
  padding: 0 1.2rem;
}

/* ================================
   Main Cards (Homepage)
=================================== */
.cards {
  display: grid;
  grid-template-columns: repeat(auto-fit, minmax(320px, 1fr)); /* bigger min width */
  gap: 1.5rem;
  align-items: stretch;
}

.card {
  position: relative;
  display: flex;
  flex-direction: column;
  gap: 0.8rem;
  padding: 1.5rem; /* more padding */
  text-decoration: none;
  color: inherit;
  border-radius: 14px;
  background: var(--card);
  border: 1px solid var(--border);
  box-shadow: var(--shadow);
  transition: transform .18s ease, box-shadow .18s ease, background .18s ease, border-color .18s ease;
}

.card:hover,
.card:focus-visible {
  background: var(--card-hover);
  border-color: var(--accent);
  box-shadow: 0 18px 44px rgba(16,46,84,0.2);
  transform: translateY(-3px);
  outline: none;
}

.card h2 {
  margin: 0 0 0.6rem 0;
  font-size: 1.5rem;   /* bigger heading */
  font-weight: 600;
  color: var(--text);
}
.card p {
  margin: 0;
  font-size: 1.1rem;   /* bigger body text */
  line-height: 1.6;
  color: var(--muted);
  flex-grow: 1;
}

/* ================================
   Responsive Tweaks
=================================== */
@media (min-width: 1400px) {
  .cards { grid-template-columns: repeat(4, 1fr); }
}

@media screen and (orientation: portrait) {
  .cards {
    grid-template-columns: 1fr;
    gap: 1.5rem;
  }
  .card {
    font-size: 1.1rem;
    padding: 1.6rem;
  }
  .card h2 { font-size: 1.6rem; }
  .card p { font-size: 1.15rem; }
}

@media screen and (orientation: landscape) and (max-width: 900px) {
  .cards {
    grid-template-columns: repeat(2, 1fr);
  }
}

@media (max-width: 640px) {
  .brand-name { display: none; }
  .nav-links { gap: 0.5rem; }
}

/* ================================
   Optional Dark Mode
=================================== */
@media (prefers-color-scheme: dark) {
  :root {
    --bg: #0b1324;
    --bg-2: #111a33;
    --card: rgba(255,255,255,0.08);
    --card-hover: rgba(255,255,255,0.12);
    --border: rgba(255,255,255,0.16);
    --text: #eef3ff;
    --muted: #b6c2e2;
    --accent: #5cc8ff;
    --accent-2: #9d7dff;
    --success: #59d38c;
    --shadow: 0 12px 32px rgba(0,0,0,0.3);
  }
  body {
    background: radial-gradient(1200px 800px at 10% 10%, #142044, var(--bg)) no-repeat,
                linear-gradient(180deg, var(--bg), var(--bg-2));
    background-attachment: fixed;
  }
  .topbar {
    background: linear-gradient(180deg, rgba(17,26,51,0.75), rgba(11,19,36,0.6));
  }
}