/*
 * Style sheet for the Bengali‑to‑Russian learning web app.
 *
 * This CSS defines a modern, responsive layout with a sticky header,
 * hero section, tables for lessons, quiz styling and advertising
 * placeholders. Colors and fonts are defined as CSS variables for
 * easy customisation. Use of a sans‑serif Bengali font ensures the
 * Bengali script renders cleanly across devices. Media queries
 * collapse navigation on small screens, making the interface
 * comfortable on mobiles.  Comments throughout explain the purpose
 * of major blocks.
 */

:root {
  /* Colour palette for the site */
  --primary-color: #243665;      /* dark blue for header/footer */
  --accent-color: #c8102e;       /* red accent reminiscent of the Russian flag */
  --background-color: #f7f7f7;   /* light background */
  --text-color: #333333;         /* standard body text */
}

body {
  margin: 0;
  font-family: 'Noto Sans Bengali', 'Noto Sans', sans-serif;
  background: var(--background-color);
  color: var(--text-color);
  line-height: 1.6;
}

/* Sticky header with site title and navigation */
header {
  background: var(--primary-color);
  color: #ffffff;
  display: flex;
  align-items: center;
  justify-content: space-between;
  padding: 10px 20px;
  flex-wrap: wrap;
  box-shadow: 0 2px 4px rgba(0,0,0,0.1);
  position: sticky;
  top: 0;
  z-index: 999;
}

header h1 {
  font-size: 1.5rem;
  margin: 0;
}

nav ul {
  list-style: none;
  margin: 0;
  padding: 0;
  display: flex;
  gap: 20px;
}

nav a {
  color: #ffffff;
  text-decoration: none;
  font-size: 1rem;
  transition: color 0.3s ease;
}

nav a:hover {
  color: var(--accent-color);
}

/* Hero section with gradient background */
.hero {
  text-align: center;
  padding: 60px 20px;
  background: linear-gradient(135deg, #e0f2f1, #e8eaf6);
}

.hero h2 {
  font-size: 2.5rem;
  margin-bottom: 20px;
  color: var(--primary-color);
}

.hero p {
  font-size: 1.2rem;
  margin-bottom: 30px;
}

.hero .cta {
  background: var(--accent-color);
  color: #ffffff;
  padding: 12px 24px;
  text-decoration: none;
  border-radius: 4px;
  font-weight: bold;
  transition: background 0.3s ease;
}

.hero .cta:hover {
  background: #a20d22;
}

/* Generic section styling */
.section {
  padding: 40px 20px;
}

/* Card grid for features */
.features {
  display: grid;
  grid-template-columns: repeat(auto-fit, minmax(250px, 1fr));
  gap: 20px;
  margin-top: 40px;
}

.feature-card {
  background: #ffffff;
  padding: 20px;
  border-radius: 8px;
  box-shadow: 0 2px 4px rgba(0,0,0,0.1);
  text-align: center;
}

.feature-card h3 {
  margin-top: 10px;
  margin-bottom: 10px;
  color: var(--primary-color);
  font-size: 1.2rem;
}

.feature-card p {
  font-size: 0.9rem;
  margin: 0;
}

/* Advertisement placeholder styling */
.ad-placeholder {
  border: 2px dashed #cccccc;
  padding: 20px;
  text-align: center;
  margin: 20px 0;
  min-height: 100px;
  border-radius: 8px;
  background: #ffffff;
  color: #888888;
  font-style: italic;
}

/* Table styling used in lessons */
.lesson-table {
  width: 100%;
  border-collapse: collapse;
  margin-top: 20px;
  margin-bottom: 30px;
}

.lesson-table th, .lesson-table td {
  border: 1px solid #dddddd;
  padding: 8px;
  text-align: left;
}

.lesson-table th {
  background: var(--primary-color);
  color: #ffffff;
}

.lesson-table td {
  background: #ffffff;
}

/* Simple button class used on practise page */
.button {
  display: inline-block;
  padding: 8px 16px;
  background: var(--primary-color);
  color: #ffffff;
  text-decoration: none;
  border-radius: 4px;
  cursor: pointer;
  border: none;
  margin-top: 10px;
  transition: background 0.3s ease;
}

.button:hover {
  background: #1a2850;
}

/* Quiz container styling for practise page */
.quiz-container {
  background: #ffffff;
  padding: 20px;
  border-radius: 8px;
  box-shadow: 0 2px 4px rgba(0,0,0,0.1);
  max-width: 600px;
  margin: 0 auto;
  text-align: center;
}

.quiz-container h3 {
  margin-bottom: 20px;
  color: var(--primary-color);
}

.quiz-options {
  display: grid;
  grid-template-columns: 1fr;
  gap: 15px;
}

.quiz-options button {
  padding: 10px;
  border: 1px solid var(--primary-color);
  border-radius: 4px;
  background: #ffffff;
  transition: background 0.3s, color 0.3s;
  cursor: pointer;
  font-size: 1rem;
}

.quiz-options button:hover {
  background: var(--primary-color);
  color: #ffffff;
}

.result {
  margin-top: 20px;
  font-weight: bold;
  font-size: 1.1rem;
}

.scoreboard {
  margin-top: 10px;
  font-weight: bold;
}

/* Footer styling */
footer {
  background: var(--primary-color);
  color: #ffffff;
  text-align: center;
  padding: 20px 10px;
  margin-top: 40px;
  font-size: 0.9rem;
}

/* Responsive navigation: collapse to column on small screens */
@media (max-width: 768px) {
  nav ul {
    flex-direction: column;
    gap: 10px;
    align-items: flex-start;
  }
  header h1 {
    font-size: 1.2rem;
    margin-bottom: 5px;
  }
  .hero h2 {
    font-size: 2rem;
  }
  .hero p {
    font-size: 1rem;
  }
}