/* =========================
   CSS Variables
========================= */
:root {
  --bg-light: #f0f2f5;
  --bg-dark: #121212;
  --container-light: #fff;
  --container-dark: #1e1e1e;
  --text-light: #333;
  --text-dark: #e0e0e0;
  --accent: #3f51b5;
  --accent-dark: #90caf9;
}

/* =========================
   Base
========================= */
body {
  font-family: 'Segoe UI', Tahoma, Geneva, Verdana, sans-serif;
  background: linear-gradient(to right, var(--bg-light), #e0e7ff);
  color: var(--text-light);
  margin: 0;
  padding: 0;
  transition: background 0.3s, color 0.3s;
}

body.dark-mode {
  background: linear-gradient(to right, var(--bg-dark), #1c1c1c);
  color: var(--text-dark);
}

/* =========================
   Container
========================= */
.container {
  max-width: 900px;
  margin: 60px auto;
  padding: 30px 20px;
  background: var(--container-light);
  box-shadow: 0 4px 12px rgba(0, 0, 0, 0.1);
  border-radius: 10px;
  transition: background 0.3s;
}

body.dark-mode .container {
  background: var(--container-dark);
  box-shadow: 0 4px 12px rgba(255, 255, 255, 0.05);
}

/* =========================
   Headings
========================= */
h1 {
  text-align: center;
  color: var(--accent);
  margin-bottom: 30px;
}

h2 {
  color: #1a237e;
  border-left: 4px solid var(--accent);
  padding-left: 12px;
  margin-top: 30px;
}

body.dark-mode h1,
body.dark-mode h2 {
  color: var(--accent-dark);
  border-color: var(--accent-dark);
}

/* =========================
   Text
========================= */
p,
li {
  font-size: 1rem;
  line-height: 1.8;
}

ul {
  padding-left: 20px;
}

/* =========================
   Links
========================= */
a {
  color: #0d47a1;
  text-decoration: none;
}

a:hover {
  text-decoration: underline;
}

body.dark-mode a {
  color: #90caf9;
}

/* =========================
   Notes
========================= */
.note {
  background: #ffeaea;
  padding: 10px 15px;
  border-left: 5px solid #d32f2f;
  color: #c62828;
  font-weight: bold;
  margin-top: 15px;
  border-radius: 6px;
}

body.dark-mode .note {
  background: #4a1c1c;
  border-left-color: #f44336;
  color: #ff8a80;
}

/* =========================
   Footer
========================= */
footer {
  text-align: center;
  margin-top: 40px;
  padding: 20px;
  font-size: 0.9em;
  color: #666;
}

body.dark-mode footer {
  color: #aaa;
}

/* =========================
   Navigation
========================= */
nav ul {
  list-style: none;
  padding: 0;
  display: flex;
  flex-wrap: wrap;
  gap: 10px;
  justify-content: center;
  margin: 20px 0;
}

nav ul li a {
  padding: 5px 10px;
  background: #e3f2fd;
  border-radius: 4px;
  font-size: 0.9em;
}

nav ul li a:hover {
  background: #bbdefb;
}

body.dark-mode nav ul li a {
  background: #263238;
  color: #90caf9;
}

body.dark-mode nav ul li a:hover {
  background: #37474f;
}

/* =========================
   Details
========================= */
details {
  margin: 15px 0;
  padding: 10px;
  background: #f5f5f5;
  border-radius: 4px;
}

body.dark-mode details {
  background: #2c2c2c;
}

details summary {
  cursor: pointer;
  font-weight: bold;
  color: #1a237e;
}

body.dark-mode details summary {
  color: #90caf9;
}

/* =========================
   Dark Mode Button
========================= */
.dark-toggle {
  position: fixed;
  top: 15px;
  right: 20px;
  background: #e3f2fd;
  color: #0d47a1;
  border: none;
  padding: 6px 10px;
  border-radius: 5px;
  cursor: pointer;
  font-size: 0.9em;
  z-index: 999;
}

.dark-toggle:hover {
  background: #bbdefb;
}

body.dark-mode .dark-toggle {
  background: #263238;
  color: #90caf9;
}

body.dark-mode .dark-toggle:hover {
  background: #37474f;
}

/* =========================
   Responsive
========================= */
@media (max-width: 600px) {
  .container {
    padding: 20px 15px;
  }

  h1 {
    font-size: 1.8rem;
  }

  h2 {
    font-size: 1.2rem;
  }
}