/* =========================================
   1. VARIABLES (BIẾN MÀU & CẤU HÌNH)
   ========================================= */
:root {
  /* Màu chủ đạo (Light Mode) */
  --primary-blue: #3b82f6;
  --primary-dark: #2563eb;

  /* Màu nền & Viền */
  --bg-page: #f0f4f8;
  --bg-card: #ffffff;
  --card-border: #d1d5db;
  --card-border-hover: #3b82f6;

  /* Màu chữ */
  --text-main: #111827;
  --text-sub: #4b5563;
  --white: #ffffff;

  /* Icon */
  --icon-bg: #f0f7ff;

  /* Shadow (Đổ bóng) */
  --shadow-card: 0 2px 4px rgba(0, 0, 0, 0.05), 0 1px 2px rgba(0, 0, 0, 0.1);
  --shadow-hover:
    0 10px 15px -3px rgba(59, 130, 246, 0.2),
    0 4px 6px -2px rgba(59, 130, 246, 0.1);
  /* Đồng bộ transition khi đổi theme */
  --theme-transition: 1.5s cubic-bezier(0.2, 0.6, 0.2, 1);
}

/* Cấu hình cho DARK MODE */
html.dark,
body.dark,
body.dark-mode {
  --bg-page: #0f172a; /* Nền tối thẫm */
  --bg-card: #1e293b; /* Card xám xanh */
  --text-main: #f1f5f9; /* Chữ trắng */
  --text-sub: #94a3b8; /* Chữ xám sáng */
  --card-border: #334155; /* Viền tối */
  --icon-bg: #334155; /* Nền icon tối */
  --shadow-card: 0 4px 6px -1px rgba(0, 0, 0, 0.5);
}

/* =========================================
   2. RESET & GLOBAL STYLES
   ========================================= */
* {
  margin: 0;
  padding: 0;
  box-sizing: border-box;
  font-family: "Segoe UI", Roboto, Helvetica, Arial, sans-serif;
}

/* Áp transition đồng bộ cho các thuộc tính màu/chuyển theme */
*,
*::before,
*::after {
  transition:
    background-color var(--theme-transition),
    color var(--theme-transition),
    border-color var(--theme-transition),
    box-shadow var(--theme-transition),
    opacity var(--theme-transition),
    filter var(--theme-transition);
}

body {
  background-color: var(--bg-page);
  color: var(--text-main);
  line-height: 1.6;
  display: flex;
  flex-direction: column;
  min-height: 100vh;
  position: relative;
}

.container {
  width: 1200px;
  max-width: 95%;
  margin: 0 auto;
}

/* =========================================
   3. ANIMATION: SUN & MOON (HIỆU ỨNG CHUYỂN CẢNH)
   ========================================= */
.theme-transition-icons {
  position: fixed;
  top: 50%;
  left: 50%;
  transform: translate(-50%, -50%);
  z-index: 9999;
  pointer-events: none; /* Không chặn click chuột */
  display: flex;
  justify-content: center;
  align-items: center;
}

#sunIcon,
#moonIcon {
  font-size: 8rem;
  position: absolute;
  opacity: 0;
  transform: translateY(100px) scale(0.5);
}

#sunIcon {
  color: #f59e0b;
  filter: drop-shadow(0 0 20px rgba(245, 158, 11, 0.5));
}

#moonIcon {
  color: #f1f5f9;
  filter: drop-shadow(0 0 20px rgba(241, 245, 249, 0.5));
}

/* Class kích hoạt animation */
.animate-rise-fade {
  animation: riseAndFade 2s cubic-bezier(0.4, 0, 0.2, 1) forwards;
}

@keyframes riseAndFade {
  0% {
    opacity: 0;
    transform: translateY(100px) scale(0.5);
  }
  30% {
    opacity: 1;
    transform: translateY(0px) scale(1.1);
  }
  60% {
    opacity: 1;
    transform: translateY(0px) scale(1);
  }
  100% {
    opacity: 0;
    transform: translateY(-50px) scale(0.9);
  }
}

/* =========================================
   4. HEADER
   ========================================= */
.main-header {
  background-color: var(--primary-blue);
  color: var(--white);
  padding: 1rem 0;
  box-shadow: 0 4px 6px -1px rgba(0, 0, 0, 0.1);
  position: sticky;
  top: 0;
  z-index: 100;
  transition: background-color var(--theme-transition);
}

body.dark-mode .main-header {
  background-color: #1e293b;
}

html.dark .main-header,
body.dark .main-header {
  background-color: #1e293b;
}

.header-flex {
  display: flex;
  justify-content: center;
  align-items: center;
  width: 100%;
  position: relative;
}

.logo a {
  color: var(--white);
  text-decoration: none;
  font-size: 1.8rem;
  font-weight: 800;
  display: flex;
  align-items: center;
  gap: 12px;
  text-transform: uppercase;
  letter-spacing: 1px;
}

/* =========================================
   5. SETTINGS BUTTON & DROPDOWN
   ========================================= */
.settings-container {
  position: absolute;
  right: 0;
  z-index: 10050; /* ensure settings sit above other fixed elements */
  pointer-events: auto; /* guard against accidental overlays */
}

.icon-btn {
  background: rgba(255, 255, 255, 0.2);
  border: none;
  color: var(--white);
  width: 40px;
  height: 40px;
  border-radius: 50%;
  cursor: pointer;
  font-size: 1.2rem;
  transition: 0.2s;
  display: flex;
  align-items: center;
  justify-content: center;
  position: relative;
  z-index: 10051; /* keep the interactive button above dropdown animation */
}

.icon-btn:hover {
  background: rgba(255, 255, 255, 0.4);
  transform: rotate(90deg);
}

.settings-dropdown {
  position: absolute;
  top: 50px;
  right: 0;
  width: 250px;
  background: var(--bg-card);
  border: 1px solid var(--card-border);
  border-radius: 8px;
  box-shadow: 0 10px 15px -3px rgba(0, 0, 0, 0.2);
  padding: 10px;
  display: none;
  z-index: 200;
}

.settings-dropdown.show {
  display: block;
  animation: fadeIn 0.2s;
}

@keyframes fadeIn {
  from {
    opacity: 0;
    transform: translateY(-10px);
  }
  to {
    opacity: 1;
    transform: translateY(0);
  }
}

.setting-item {
  display: flex;
  justify-content: space-between;
  align-items: center;
  padding: 8px;
  color: var(--text-main);
  font-weight: 500;
}

/* --- Toggle Switch (Nút gạt) --- */
.switch {
  position: relative;
  display: inline-block;
  width: 40px;
  height: 22px;
}

.switch input {
  opacity: 0;
  width: 0;
  height: 0;
}

.slider {
  position: absolute;
  cursor: pointer;
  top: 0;
  left: 0;
  right: 0;
  bottom: 0;
  background-color: #ccc;
  transition: 0.4s;
  border-radius: 34px;
}

.slider:before {
  position: absolute;
  content: "";
  height: 16px;
  width: 16px;
  left: 3px;
  bottom: 3px;
  background-color: white;
  transition: 0.4s;
  border-radius: 50%;
}

input:checked + .slider {
  background-color: var(--primary-blue);
}

input:checked + .slider:before {
  transform: translateX(18px);
}

/* =========================================
   6. HERO BANNER
   ========================================= */
.hero-banner {
  position: relative;
  color: var(--white);
  text-align: center;
  padding: 3.5rem 1rem;
  margin-bottom: 2.5rem;
  clip-path: polygon(0 0, 100% 0, 100% 90%, 50% 100%, 0 90%);
  overflow: hidden;
}

/* Cross-fade two layered backgrounds so gradient transition eases smoothly */
.hero-banner::before,
.hero-banner::after {
  content: "";
  position: absolute;
  inset: 0;
  z-index: -1;
  transition: opacity var(--theme-transition);
  will-change: opacity;
}

/* Light theme background (visible by default) */
.hero-banner::before {
  background: linear-gradient(135deg, #2563eb, #60a5fa);
  opacity: 1;
}

/* Dark theme background (fades in when dark) */
.hero-banner::after {
  background: linear-gradient(135deg, #1e293b, #334155);
  opacity: 0;
}

/* When dark class is set, cross-fade backgrounds */
html.dark .hero-banner::before,
body.dark .hero-banner::before {
  opacity: 0;
}

html.dark .hero-banner::after,
body.dark .hero-banner::after {
  opacity: 1;
}

.hero-banner h1 {
  font-size: 2.5rem;
  margin-bottom: 0.8rem;
  font-weight: 800;
  text-shadow: 0 2px 4px rgba(0, 0, 0, 0.1);
}

.hero-banner p {
  font-size: 1.2rem;
  opacity: 0.95;
  font-weight: 500;
}

/* =========================================
   7. TOPIC GRID & CARDS
   ========================================= */
.section-heading {
  text-align: center;
  font-size: 2rem;
  margin-bottom: 2.5rem;
  color: var(--text-main);
  position: relative;
  font-weight: 700;
  transition: color 1.5s ease;
}

.section-heading::after {
  content: "";
  display: block;
  width: 80px;
  height: 5px;
  background: var(--primary-blue);
  margin: 10px auto 0;
  border-radius: 10px;
}

.topic-grid {
  display: grid;
  grid-template-columns: repeat(auto-fill, minmax(320px, 1fr));
  gap: 30px;
  padding-bottom: 60px;
}

/* --- Card Style --- */
.topic-card {
  background: var(--bg-card);
  color: var(--text-main);
  border: 2px solid var(--card-border);
  box-shadow: var(--shadow-card);
  border-radius: 16px;
  padding: 35px 25px;
  text-decoration: none;
  display: flex;
  flex-direction: column;
  align-items: center;
  text-align: center;
  position: relative;
  overflow: hidden;
  /* Transition chậm cho màu nền, nhanh cho hover */
  transition:
    background-color 1.5s ease,
    border-color 1.5s ease,
    color 1.5s ease,
    box-shadow 0.3s ease,
    transform 0.3s ease;
}

.topic-card:hover {
  transform: translateY(-8px);
  border-color: var(--primary-blue);
  box-shadow:
    0 10px 15px -3px rgba(59, 130, 246, 0.2),
    0 4px 6px -2px rgba(59, 130, 246, 0.1);
}

/* Thanh màu trên đầu card */
.topic-card::before {
  content: "";
  position: absolute;
  top: 0;
  left: 0;
  right: 0;
  height: 6px;
  background: var(--card-border);
  transition: background-color 1.5s ease;
}

.topic-card:hover::before {
  background: var(--primary-blue);
  transition: 0.3s ease;
}

/* --- Icon Box --- */
.icon-box {
  width: 80px;
  height: 80px;
  border-radius: 50%;
  background-color: var(--icon-bg);
  color: var(--primary-blue);
  display: flex;
  align-items: center;
  justify-content: center;
  font-size: 2rem;
  margin-bottom: 1.5rem;
  border: 1px solid var(--card-border);
  transition:
    background-color 1.5s ease,
    border-color 1.5s ease,
    color 0.3s ease,
    transform 0.3s ease;
}

.topic-card:hover .icon-box {
  background-color: var(--primary-blue);
  color: var(--white);
  border-color: var(--primary-blue);
  transform: rotateY(180deg);
}

.topic-card:hover .icon-box i {
  transform: rotateY(-180deg);
  transition: 0.3s;
}

.topic-card h3 {
  font-size: 1.35rem;
  font-weight: 700;
  margin-bottom: 1rem;
  color: var(--text-main);
  transition: color 1.5s ease;
}

.topic-card p {
  font-size: 1rem;
  color: var(--text-sub);
  margin-bottom: 0;
  line-height: 1.5;
  transition: color 1.5s ease;
}

/* =========================================
   8. FOOTER
   ========================================= */
.main-footer {
  margin-top: auto;
  background: #f0f4f8;
  color: #4b5563;
  text-align: center;
  padding: 25px;
  font-size: 0.95rem;
  border-top: 4px solid var(--primary-blue);
  transition:
    background-color 1.5s ease,
    color 1.5s ease,
    border-top-color 1.5s ease;
}

body.dark-mode .main-footer,
html.dark .main-footer,
body.dark .main-footer {
  background: #0f172a;
  color: #94a3b8;
  border-top-color: #60a5fa;
}