:root {
      --primary-color: #4a6cf7;
      --secondary-color: #f76c6c;
      --tertiary-color: #6cf7a4;
      --background-color: #f5f7fa;
      --text-color: #333;
      --border-radius: 8px;
      --box-shadow: 0 4px 6px rgba(0, 0, 0, 0.1);
      --transition-speed: 0.3s;
    }

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

    body {
      font-family: 'Segoe UI', Tahoma, Geneva, Verdana, sans-serif;
      background-color: var(--background-color);
      color: var(--text-color);
      line-height: 1.6;
      padding: 20px;
      transition: background-color var(--transition-speed);
    }

    header {
      text-align: center;
      margin-bottom: 20px;
    }

    h1 {
      color: var(--primary-color);
      margin-bottom: 10px;
      font-size: 2.2rem;
    }

    .subheading {
      color: var(--secondary-color);
      font-size: 1.2rem;
      margin-bottom: 20px;
    }

    .game-container {
      max-width: 1200px;
      margin: 0 auto;
      display: grid;
      grid-template-columns: 1fr 3fr;
      gap: 20px;
    }

    @media (max-width: 768px) {
      .game-container {
        grid-template-columns: 1fr;
      }
    }

    .sidebar {
      background-color: white;
      border-radius: var(--border-radius);
      padding: 20px;
      box-shadow: var(--box-shadow);
    }

    .sidebar h2 {
      color: var(--primary-color);
      margin-bottom: 15px;
      font-size: 1.5rem;
      border-bottom: 2px solid var(--primary-color);
      padding-bottom: 5px;
    }

    .game-info {
      margin-bottom: 30px;
    }

    .badges-container {
      display: flex;
      flex-direction: column;
      gap: 15px;
    }

    .badge {
      display: flex;
      align-items: center;
      padding: 10px;
      background-color: #f0f2f5;
      border-radius: var(--border-radius);
      opacity: 0.7;
      transition: all var(--transition-speed);
    }

    .badge.achieved {
      background-color: #e6f7ef;
      opacity: 1;
      box-shadow: 0 2px 4px rgba(0, 150, 0, 0.1);
    }

    .badge-icon {
      width: 40px;
      height: 40px;
      background-color: #ddd;
      border-radius: 50%;
      display: flex;
      justify-content: center;
      align-items: center;
      margin-right: 10px;
      font-size: 1.2rem;
      color: #555;
    }

    .badge.achieved .badge-icon {
      background-color: var(--tertiary-color);
      color: #333;
    }

    .badge-info {
      flex-grow: 1;
    }

    .badge-name {
      font-weight: bold;
      color: #444;
    }

    .badge-desc {
      font-size: 0.85rem;
      color: #666;
    }

    .main-content {
      background-color: white;
      border-radius: var(--border-radius);
      padding: 20px;
      box-shadow: var(--box-shadow);
    }

    .actions {
      display: flex;
      justify-content: space-between;
      margin-bottom: 20px;
      flex-wrap: wrap;
      gap: 10px;
    }

    .btn {
      padding: 10px 20px;
      border: none;
      border-radius: var(--border-radius);
      font-size: 1rem;
      cursor: pointer;
      transition: all var(--transition-speed);
    }

    .btn-primary {
      background-color: var(--primary-color);
      color: white;
    }

    .btn-primary:hover {
      background-color: #3a5ce5;
      transform: translateY(-2px);
    }

    .btn-secondary {
      background-color: #e0e0e0;
      color: #333;
    }

    .btn-secondary:hover {
      background-color: #d0d0d0;
      transform: translateY(-2px);
    }

    .btn-danger {
      background-color: var(--secondary-color);
      color: white;
    }

    .btn.btn-disabled {
      opacity: 0.5;
      cursor: not-allowed;
      pointer-events: none;
    }

    .level-selector {
      display: flex;
      justify-content: center;
      margin-bottom: 20px;
    }

    .level-btn {
      background-color: #e0e0e0;
      color: #333;
      padding: 8px 16px;
      margin: 0 5px;
      border: none;
      border-radius: var(--border-radius);
      cursor: pointer;
      transition: all var(--transition-speed);
    }

    .level-btn:hover {
      background-color: #d0d0d0;
    }

    .level-btn.active {
      background-color: var(--primary-color);
      color: white;
    }

    .timer-container {
      text-align: center;
      margin-bottom: 20px;
      font-size: 1.5rem;
      font-weight: bold;
      color: var(--primary-color);
    }

    .periodic-table {
      display: grid;
      grid-template-columns: repeat(4, 1fr);
      gap: 10px;
      margin: 0 auto;
      max-width: 600px;
    }

    @media (min-width: 768px) {
      .periodic-table.medium {
        grid-template-columns: repeat(5, 1fr);
      }

      .periodic-table.hard {
        grid-template-columns: repeat(6, 1fr);
      }
    }

    .element {
      position: relative;
      aspect-ratio: 1/1;
      border-radius: var(--border-radius);
      background-color: white;
      box-shadow: var(--box-shadow);
      display: flex;
      flex-direction: column;
      justify-content: center;
      align-items: center;
      cursor: pointer;
      user-select: none;
      transition: all var(--transition-speed);
      overflow: hidden;
    }

    .element:hover {
      transform: translateY(-4px);
      box-shadow: 0 6px 12px rgba(0, 0, 0, 0.15);
    }

    .element.selected {
      background-color: var(--tertiary-color);
    }

    .element.selected::before {
      content: "✓";
      position: absolute;
      top: 5px;
      right: 5px;
      font-size: 1rem;
      color: green;
    }

    .element-name {
      font-weight: bold;
      text-align: center;
      font-size: 0.9rem;
    }

    .element-symbol {
      display: none;
      font-size: 1.2rem;
      font-weight: bold;
      margin-bottom: 5px;
    }

    .element.symbol-mode .element-name {
      display: none;
    }

    .element.symbol-mode .element-symbol {
      display: block;
    }

    /* Tooltip */
    .tooltip {
      position: absolute;
      bottom: 100%;
      left: 50%;
      transform: translateX(-50%);
      padding: 8px 12px;
      background-color: rgba(0, 0, 0, 0.8);
      color: white;
      border-radius: 4px;
      font-size: 0.8rem;
      pointer-events: none;
      opacity: 0;
      transition: opacity var(--transition-speed);
      width: max-content;
      max-width: 200px;
      z-index: 100;
    }

    .element:hover .tooltip {
      opacity: 1;
    }

    /* Modal */
    .modal {
      display: none;
      position: fixed;
      z-index: 999;
      left: 0;
      top: 0;
      width: 100%;
      height: 100%;
      overflow: auto;
      background-color: rgba(0, 0, 0, 0.7);
      animation: fadeIn 0.3s;
    }

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

    .modal-content {
      background-color: white;
      margin: 10% auto;
      padding: 30px;
      border-radius: var(--border-radius);
      box-shadow: var(--box-shadow);
      max-width: 500px;
      text-align: center;
      position: relative;
      animation: slideIn 0.4s;
    }

    @keyframes slideIn {
      from { transform: translateY(-50px); opacity: 0; }
      to { transform: translateY(0); opacity: 1; }
    }

    .close-modal {
      position: absolute;
      top: 10px;
      right: 15px;
      font-size: 1.5rem;
      font-weight: bold;
      cursor: pointer;
    }

    .modal-title {
      color: var(--primary-color);
      margin-bottom: 15px;
      font-size: 1.8rem;
    }

    .modal-image {
      max-width: 100%;
      height: auto;
      margin: 15px 0;
      border-radius: var(--border-radius);
    }

    .stats-container {
      display: grid;
      grid-template-columns: 1fr 1fr;
      gap: 15px;
      margin: 20px 0;
    }

    .stat-item {
      background-color: #f5f7fa;
      padding: 15px;
      border-radius: var(--border-radius);
      text-align: center;
    }

    .stat-value {
      font-size: 1.8rem;
      font-weight: bold;
      color: var(--primary-color);
    }

    .stat-label {
      font-size: 0.9rem;
      color: #777;
    }

    .confetti {
      position: absolute;
      width: 10px;
      height: 10px;
      background-color: #f00;
      border-radius: 50%;
      animation: confetti-fall 5s linear forwards;
    }

    @keyframes confetti-fall {
      0% { transform: translateY(-10vh) rotate(0deg); opacity: 1; }
      100% { transform: translateY(100vh) rotate(720deg); opacity: 0; }
    }

    .progress-container {
      height: 10px;
      background-color: #e0e0e0;
      border-radius: 10px;
      margin: 15px 0;
      overflow: hidden;
    }

    .progress-bar {
      height: 100%;
      background-color: var(--primary-color);
      border-radius: 10px;
      transition: width 0.5s;
      width: 0%;
    }

    /* Theme toggle */
    .theme-toggle {
      position: fixed;
      bottom: 20px;
      right: 20px;
      width: 50px;
      height: 50px;
      border-radius: 50%;
      background-color: var(--primary-color);
      display: flex;
      justify-content: center;
      align-items: center;
      cursor: pointer;
      box-shadow: var(--box-shadow);
      transition: all var(--transition-speed);
    }

    .theme-toggle:hover {
      transform: scale(1.1);
    }

    .theme-toggle i {
      color: white;
      font-size: 1.5rem;
    }

    /* Dark theme */
    body.dark-theme {
      background-color: #1a1a2e;
      color: #f0f0f0;
    }

    body.dark-theme .sidebar,
    body.dark-theme .main-content,
    body.dark-theme .modal-content {
      background-color: #16213e;
      color: #f0f0f0;
    }

    body.dark-theme .element {
      background-color: #16213e;
      border: 1px solid #444;
    }

    body.dark-theme .element-name {
      color: #f0f0f0;
    }

    body.dark-theme .btn-secondary {
      background-color: #444;
      color: #f0f0f0;
    }

    body.dark-theme .element.selected {
      background-color: #1e5128;
    }

    body.dark-theme .badge {
      background-color: #1e2a3a;
    }

    body.dark-theme .badge.achieved {
      background-color: #1e3a2a;
    }

    /* Help section */
    .help-container {
      margin-top: 20px;
      padding: 15px;
      background-color: #f0f2f5;
      border-radius: var(--border-radius);
    }

    body.dark-theme .help-container {
      background-color: #1e2a3a;
    }

    .help-container h3 {
      margin-bottom: 10px;
      color: var(--primary-color);
    }

    .help-container p {
      margin-bottom: 10px;
      font-size: 0.9rem;
    }