/* Reset and Base Styles */
:root {
  --primary-color: #0a2540;
  --secondary-color: #f0f4f8;
  --accent-color: #1a73e8;
  --text-color: #1e293b;
  --text-light: #64748b;
  --heading-color: #0a2540;
  --background-color: #ffffff;
  --bg-light: #f8fafc;
  --border-color: #e2e8f0;
  --heading-font: "Roboto", "Inter", sans-serif;
  --body-font: "Open Sans", "Lato", sans-serif;
  --logo-font: "Georgia", serif;
  --mono-font: "JetBrains Mono", "Fira Code", monospace;
  --border-radius: 8px;
  --box-shadow: 0 4px 12px rgba(0, 0, 0, 0.08);
  --transition: all 0.3s ease;
  --container-width: 1200px;
}

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

body {
  font-family: var(--body-font);
  color: var(--text-color);
  background-color: var(--background-color);
  line-height: 1.7;
}

/* Typography */
h1,
h2,
h3,
h4,
h5,
h6 {
  font-family: var(--heading-font);
  color: var(--heading-color);
  font-weight: 700;
  margin-bottom: 0.75em;
  letter-spacing: -0.01em;
}

h1 {
  font-size: 2.5rem;
}

h2 {
  font-size: 2.2rem;
}

h3 {
  font-size: 1.5rem;
}

h4 {
  font-size: 1.2rem;
}

p {
  font-size: 0.95rem;
  text-align: justify;
  margin-bottom: 1rem;
  color: #374151;
}

a {
  color: var(--accent-color);
  text-decoration: none;
  transition: var(--transition);
}

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

/* Layout */
/* Layout & Sidebar System */
body {
  display: flex;
  min-height: 100vh;
  background-color: #f8f9fa;
  /* Light grey background for the app */
}

/* Sidebar (Left Navigation) */
.sidebar {
  width: 280px;
  background-color: #fafbfc;
  border-right: 1px solid #e8ecf1;
  height: 100vh;
  position: fixed;
  top: 0;
  left: 0;
  overflow-y: auto;
  z-index: 1000;
  padding: 2.5rem 1.5rem;
  display: flex;
  flex-direction: column;
}

.sidebar-header {
  margin-bottom: 3rem;
}

.sidebar-logo {
  display: block;
  text-decoration: none;
}

.sidebar-logo h1 {
  font-family: var(--logo-font);
  font-size: 2.45rem;
  line-height: 1.2;
  margin-bottom: 0.5rem;
  color: var(--primary-color);
  letter-spacing: 0;
  font-weight: 800;
  font-kerning: normal;
  font-stretch: normal;
  font-feature-settings: "kern" 1, "liga" 1;
  text-rendering: geometricPrecision;
}

.sidebar-subtitle {
  font-family: 'Inter', sans-serif;
  font-size: 0.98rem;
  color: #64748b;
  font-weight: 700;
  line-height: 1.4;
  display: block;
  letter-spacing: 0.02em;
}

/* Sidebar Navigation */
.sidebar-nav {
  flex: 1;
  display: flex;
  flex-direction: column;
  gap: 0.25rem;
}

.nav-item {
  display: block;
  padding: 0.55rem 1rem;
  border-radius: 6px;
  color: #475569;
  font-family: 'Inter', sans-serif;
  font-weight: 500;
  font-size: 0.9rem;
  transition: all 0.2s ease;
  border-left: 3px solid transparent;
  letter-spacing: 0.01em;
}

.nav-item:hover {
  background-color: #f1f5f9;
  color: var(--primary-color);
}

.nav-item.active {
  background-color: rgba(10, 37, 64, 0.04);
  color: var(--primary-color);
  border-left: 3px solid var(--accent-color);
  font-weight: 600;
}

.sidebar-submenu {
  display: none;
  padding-left: 1.5rem;
  flex-direction: column;
  gap: 0.2rem;
  margin-bottom: 0.5rem;
}

.sidebar-submenu.open {
  display: flex;
}

/* Sidebar Footer (Lang & Copyright) */
.sidebar-footer {
  margin-top: auto;
  padding-top: 2rem;
  border-top: 1px solid #e2e8f0;
}

.lang-switch {
  margin-bottom: 1rem;
  font-size: 1.7rem;
}

.copyright {
  font-size: 0.75rem;
  color: #94a3b8;
  line-height: 1.5;
}

/* Main Content Area */
.main-content {
  flex: 1;
  margin-left: 280px;
  width: calc(100% - 280px);
  /* Width of sidebar */
  min-width: 0;
  /* Prevent flex overflow */
  display: flex;
  flex-direction: column;
  background-color: #fff;
}

.main-content > .container {
  max-width: 1200px;
  margin: 0 auto;
  padding: 4rem 3rem;
  width: 100%;
}

/* Challenges Section (KAIST Style) */
.challenges-list {
  display: flex;
  flex-direction: column;
  gap: 4rem;
  /* Spacing between challenges */
}

.challenge-item {
  display: grid;
  grid-template-columns: 1fr 1fr;
  /* Text | Image */
  gap: 3rem;
  align-items: start;
  padding-bottom: 4rem;
  border-bottom: 1px solid #e2e8f0;
}

.challenge-item:last-child {
  border-bottom: none;
}

.challenge-item.reverse {
  direction: rtl;
  /* Flip order */
}

.challenge-item.reverse .challenge-text {
  direction: ltr;
  /* Restore text direction */
}

.challenge-text h3 {
  font-size: 2rem;
  color: var(--heading-color);
  margin-bottom: 1.5rem;
  line-height: 1.2;
}

.challenge-text p {
  font-size: 1.05rem;
  color: #475569;
  line-height: 1.8;
  margin-bottom: 2rem;
}

.challenge-media {
  width: 100%;
  background-color: #f8fafc;
  border-radius: 12px;
  overflow: hidden;
  position: relative;
  border: 1px solid #e2e8f0;
  min-height: 350px;
  /* Placeholder height */
  display: flex;
  align-items: center;
  justify-content: center;
}

.challenge-media img {
  width: 100%;
  height: auto;
  display: block;
}

.challenge-placeholder {
  color: #94a3b8;
  font-weight: 500;
  text-align: center;
  padding: 2rem;
}

/* Responsive Design (Mobile) */
@media (max-width: 1024px) {
  .sidebar {
    width: 100%;
    height: auto;
    position: relative;
    border-right: none;
    border-bottom: 1px solid #e2e8f0;
  }

  .main-content {
    margin-left: 0;
  }

  .challenge-item {
    grid-template-columns: 1fr;
    /* Stack vertically */
  }
}

/* Responsive Design */
@media (max-width: 768px) {
  h1 {
    font-size: 2.5rem;
  }

  h2 {
    font-size: 2rem;
  }

  .nav-links {
    display: none;
    /* Implement mobile menu logic if needed */
  }

  /* .hero {} removed */

  .hero h2 {
    font-size: 2.2rem;
  }

  .hero p {
    font-size: 1.1rem;
    padding: 0 10px;
  }

  /* Footer */
  footer .container {
    flex-direction: column;
    gap: 20px;
    text-align: center;
  }

  footer a {
    margin: 0 10px;
    display: inline-block;
  }

  /* Stats */
  .stat-item h3 {
    font-size: 2.5rem;
  }
}
/* Sidebar Submenu Styles */
.sidebar-submenu .nav-item {
  font-size: 0.9rem;
  padding: 0.4rem 1rem;
  color: #64748b;
  font-weight: 400;
  border-left: 3px solid transparent !important; /* Ensure consistent spacing */
}

.sidebar-submenu .nav-item:hover {
  color: var(--primary-color);
  background-color: rgba(0, 51, 102, 0.03);
}

.sidebar-submenu .nav-item.active {
  color: var(--primary-color);
  background-color: rgba(0, 51, 102, 0.05);
  font-weight: 600;
  border-left: 3px solid transparent !important; /* No border for submenu items */
}