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

body {
  font-family: 'Poppins', sans-serif;
  background: linear-gradient(135deg, #f5f7fa 0%, #d4ddea 70%, #c3cfe2 100%);
  color: #333;
  overflow-x: hidden;
  transition: background 0.5s ease;
}

/* === NAV BAR === */
nav {
  position: fixed;
  top: 0;
  left: 0;
  width: 100%;
  background: linear-gradient(135deg, #d4ddea 0%, #b0c4de 100%);
  color: #333;
  display: flex;
  align-items: center;
  justify-content: space-between;
  padding: 15px 30px;
  z-index: 1000;
  box-shadow: 0 4px 10px rgba(0, 0, 0, 0.1);
}

.nav-title {
  font-size: 1.5rem;
  font-weight: 700;
  text-align: center;
  flex-grow: 1;
  letter-spacing: 1.5px;
  color: #2b3a55;
  text-shadow: 1px 1px 2px rgba(0, 0, 0, 0.1);
}

.menu-toggle {
  cursor: pointer;
  display: flex;
  flex-direction: column;
  justify-content: space-between;
  width: 30px;
  height: 20px;
}

.menu-toggle span {
  background: #2b3a55;
  height: 3px;
  border-radius: 2px;
  transition: all 0.3s ease;
}

.menu-toggle.active span:nth-child(1) {
  transform: rotate(45deg) translate(5px, 5px);
}

.menu-toggle.active span:nth-child(2) {
  opacity: 0;
}

.menu-toggle.active span:nth-child(3) {
  transform: rotate(-45deg) translate(7px, -7px);
}
/* search box */
.search-container {
    position: relative;
    display: flex;
    align-items: center;
    background: rgb(179, 186, 223);
    border-radius: 25px;
    padding: 5px 12px;
    box-shadow: 0 2px 8px rgb(43 156 239 / 100%);
    transition: all 0.4s ease;
    width: 100%;
    margin-bottom: 50px;
}

.search-container:focus-within {
  background: #fff;
  box-shadow: 0 4px 12px rgba(0, 0, 0, 0.15);
  transform: scale(1.05);
}

#searchInput {
  border: none;
  background: transparent;
  outline: none;
  width: 100%;
  padding: 8px 10px 8px 5px;
  font-size: 0.95rem;
  color: #2b3a55;
  font-family: 'Poppins', sans-serif;
}

#searchInput::placeholder {
  color: #4d4961;
  font-style: italic;
  font-weight: 700;
  font-size: 1rem;
}

.search-icon {
  width: 18px;
  height: 18px;
  fill: #2b3a55;
  flex-shrink: 0;
}

/* Ẩn tiêu đề khi tìm kiếm (tùy chọn, nếu muốn gọn hơn trên mobile) */
@media (max-width: 768px) {
  .search-container {
    padding: 4px 10px;
  }

  #searchInput {
    font-size: 0.9rem;
  }
}

@media (max-width: 480px) {


  .nav-title {
    font-size: 1.1rem;
  }
}

/* === SIDEBAR === */
.sidebar {
  position: fixed;
  top: 70px;
  left: 0;
  width: 280px;
  height: calc(100vh - 70px);
  background: rgba(255, 255, 255, 0.95);
  padding: 20px;
  transform: translateX(-100%);
  transition: transform 0.4s cubic-bezier(0.68, -0.55, 0.27, 1.55);
  box-shadow: 4px 0 15px rgba(0, 0, 0, 0.1);
  overflow-y: auto;
}

.sidebar.open {
  transform: translateX(0);
  z-index:1;
}

.sidebar h3 {
  font-size: 1.4rem;
  color: #2b3a55;
  margin-bottom: 20px;
  text-align: center;
  animation: fadeIn 0.5s ease;
}

.lesson {
  display: flex;
  align-items: center;
  padding: 12px 15px;
  margin: 10px 0;
  background: linear-gradient(135deg, #fff, #e6eef8);
  border-radius: 15px;
  cursor: pointer;
  transition: all 0.3s ease;
  box-shadow: 0 3px 8px rgba(0, 0, 0, 0.05);
  animation: slideIn 0.5s ease forwards;
}

.lesson:hover {
  background: linear-gradient(135deg, #b0c4de, #d4ddea);
  color: #2b3a55;
  transform: scale(1.05) rotate(2deg);
  box-shadow: 0 5px 15px rgba(0, 0, 0, 0.15);
}

.lesson.active {
  background: linear-gradient(135deg, #2b3a55, #4a6fa5);
  color: #fff;
  transform: scale(1.05);
}

.lesson span {
  font-size: 1rem;
  font-weight: 600;
}

@keyframes slideIn {
  from { opacity: 0; transform: translateX(-20px); }
  to { opacity: 1; transform: translateX(0); }
}

@keyframes fadeIn {
  from { opacity: 0; }
  to { opacity: 1; }
}

/* === MAIN === */
main {
  margin-top: 70px;
  padding: 20px;
  transition: margin-left 0.4s ease;
  display: flex;
  justify-content: center;
  align-items: center;
  min-height: calc(100vh - 70px);
}

.sidebar.open ~ main {
  margin-left: 280px;
}

.mindmap-container {
  position: relative;
  max-width: 80%;
  text-align: center;
  cursor: zoom-in;
  transition: transform 0.3s ease;
}

.mindmap-image {
  max-width: 100%;
  border-radius: 20px;
  box-shadow: 0 10px 25px rgba(0, 0, 0, 0.15);
  transition: transform 0.3s ease, opacity 0.3s ease;
  opacity: 0;
  transform: scale(0.95);
}

.mindmap-image.loaded {
  opacity: 1;
  transform: scale(1);
}

.mindmap-container:hover .mindmap-image {
  transform: scale(1.02);
}

.image-overlay {
  position: absolute;
  top: 20px;
  left: 20px;
  background: rgba(43, 58, 85, 0.8);
  padding: 10px 20px;
  border-radius: 10px;
  color: #fff;
  box-shadow: 0 2px 5px rgba(0, 0, 0, 0.2);
}

.image-overlay h2 {
  font-size: 1.4rem;
  font-weight: 700;
}

/* === MODAL === */
.modal {
  display: none;
  position: fixed;
  z-index: 9999;
  padding-top: 60px;
  left: 0;
  top: 0;
  width: 100%;
  height: 100%;
  overflow: auto;
  background-color: rgba(0, 0, 0, 0.9);
  cursor: move;
}

.modal-content {
  margin: auto;
  display: block;
  width: 80%;
  max-width: 1200px;
  transition: transform 0.3s ease;
  transform-origin: center;
  position: relative;
}

.close {
position: absolute;
    top: 15px;
    right: 35px;
    color: #892525;
    font-size: 70px;
    font-weight: bold;
    transition: 0.3s;
    z-index: 11;
}

.close:hover,
.close:focus {
  color: #bbb;
  text-decoration: none;
  cursor: pointer;
}

/* === BACK BUTTON === */
.back-btn {
  position: fixed;
  bottom: 30px;
  right: 30px;
  width: 60px;
  height: 60px;
  border-radius: 50%;
  background: linear-gradient(135deg, #2b3a55, #4a6fa5);
  border: none;
  box-shadow: 0 8px 25px rgba(0, 0, 0, 0.2);
  cursor: pointer;
  display: flex;
  align-items: center;
  justify-content: center;
  transition: transform 0.2s, box-shadow 0.3s;
  z-index: 9999;
}

.back-btn:hover {
  transform: translateY(-5px) scale(1.05);
  box-shadow: 0 12px 35px rgba(0, 0, 0, 0.3);
}

.back-btn:active {
  transform: scale(0.95);
}

.back-btn svg {
  width: 28px;
  height: 28px;
  fill: #fff;
}

/* === SCROLLBAR === */
::-webkit-scrollbar {
  width: 8px;
}

::-webkit-scrollbar-track {
  background: #e6eef8;
}

::-webkit-scrollbar-thumb {
  background: #2b3a55;
  border-radius: 10px;
}

/* === RESPONSIVE === */
@media (max-width: 768px) {
  .sidebar {
    width: 100%;
    top: 60px;
    height: calc(100vh - 60px);
    padding: 15px;
  }

  .sidebar.open ~ main {
    margin-left: 0;
  }

  .mindmap-container {
    max-width: 100%;
  }

  .nav-title {
    font-size: 1.2rem;
  }

  .modal-content {
    width: 95%;
  }
}

@media (max-width: 480px) {
  nav {
    padding: 10px 20px;
  }

  .lesson {
    padding: 10px;
    font-size: 0.9rem;
  }

  .mindmap-container {
    max-width: 100%;
  }
}