/* Reset and Base Styles */
* {
    margin: 0;
    padding: 0;
    box-sizing: border-box;
  }
  
  :root {
    --primary-color: #6366f1;
    --primary-hover: #5855eb;
    --secondary-color: #64748b;
    --success-color: #10b981;
    --warning-color: #f59e0b;
    --error-color: #ef4444;
    --background: #ffffff;
    --surface: #f8fafc;
    --surface-hover: #f1f5f9;
    --text-primary: #1e293b;
    --text-secondary: #64748b;
    --text-muted: #94a3b8;
    --border: #e2e8f0;
    --border-hover: #cbd5e1;
    --shadow-sm: 0 1px 2px 0 rgb(0 0 0 / 0.05);
    --shadow: 0 1px 3px 0 rgb(0 0 0 / 0.1), 0 1px 2px -1px rgb(0 0 0 / 0.1);
    --shadow-lg: 0 10px 15px -3px rgb(0 0 0 / 0.1), 0 4px 6px -4px rgb(0 0 0 / 0.1);
    --radius: 8px;
    --radius-lg: 12px;
    --font-family: "Inter", -apple-system, BlinkMacSystemFont, "Segoe UI", Roboto, sans-serif;
  }
  
  [data-theme="dark"] {
    --background: #0f172a;
    --surface: #1e293b;
    --surface-hover: #334155;
    --text-primary: #f1f5f9;
    --text-secondary: #cbd5e1;
    --text-muted: #94a3b8;
    --border: #334155;
    --border-hover: #475569;
  }
  
  body {
    font-family: var(--font-family);
    background: var(--background);
    color: var(--text-primary);
    line-height: 1.6;
    transition: background-color 0.3s ease, color 0.3s ease;
  }
  
  /* Loading Overlay */
  .loading-overlay {
    position: fixed;
    top: 0;
    left: 0;
    width: 100%;
    height: 100%;
    background: rgba(255, 255, 255, 0.95);
    display: flex;
    flex-direction: column;
    align-items: center;
    justify-content: center;
    z-index: 9999;
    backdrop-filter: blur(4px);
  }
  
  [data-theme="dark"] .loading-overlay {
    background: rgba(15, 23, 42, 0.95);
  }
  
  .loading-spinner {
    width: 40px;
    height: 40px;
    border: 4px solid var(--border);
    border-top: 4px solid var(--primary-color);
    border-radius: 50%;
    animation: spin 1s linear infinite;
    margin-bottom: 1rem;
  }
  
  @keyframes spin {
    0% {
      transform: rotate(0deg);
    }
    100% {
      transform: rotate(360deg);
    }
  }
  
  /* Login Styles */
  .login-container {
    min-height: 100vh;
    display: flex;
    align-items: center;
    justify-content: center;
    background: linear-gradient(135deg, var(--primary-color) 0%, #8b5cf6 100%);
    padding: 1rem;
  }
  
  .login-card {
    background: var(--background);
    padding: 2.5rem;
    border-radius: var(--radius-lg);
    box-shadow: var(--shadow-lg);
    width: 100%;
    max-width: 400px;
    animation: slideUp 0.5s ease-out;
  }
  
  @keyframes slideUp {
    from {
      opacity: 0;
      transform: translateY(20px);
    }
    to {
      opacity: 1;
      transform: translateY(0);
    }
  }
  
  .login-header {
    text-align: center;
    margin-bottom: 2rem;
  }
  
  .login-header h2 {
    font-size: 1.75rem;
    font-weight: 700;
    margin-bottom: 0.5rem;
    color: var(--text-primary);
  }
  
  .login-header p {
    color: var(--text-secondary);
    font-size: 0.95rem;
  }
  
  .input-group {
    position: relative;
    margin-bottom: 1.5rem;
  }
  
  .input-group input {
    width: 100%;
    padding: 0.875rem 1rem 0.875rem 3rem;
    border: 2px solid var(--border);
    border-radius: var(--radius);
    font-size: 1rem;
    background: var(--background);
    color: var(--text-primary);
    transition: all 0.2s ease;
  }
  
  .input-group input:focus {
    outline: none;
    border-color: var(--primary-color);
    box-shadow: 0 0 0 3px rgb(99 102 241 / 0.1);
  }
  
  .input-icon {
    position: absolute;
    left: 1rem;
    top: 50%;
    transform: translateY(-50%);
    font-size: 1.1rem;
    opacity: 0.6;
  }
  
  .login-btn {
    width: 100%;
    padding: 0.875rem;
    background: var(--primary-color);
    color: white;
    border: none;
    border-radius: var(--radius);
    font-size: 1rem;
    font-weight: 600;
    cursor: pointer;
    transition: all 0.2s ease;
    position: relative;
    overflow: hidden;
  }
  
  .login-btn:hover {
    background: var(--primary-hover);
    transform: translateY(-1px);
    box-shadow: var(--shadow);
  }
  
  .login-btn:active {
    transform: translateY(0);
  }
  
  .btn-loader {
    display: none;
    width: 20px;
    height: 20px;
    border: 2px solid transparent;
    border-top: 2px solid white;
    border-radius: 50%;
    animation: spin 1s linear infinite;
    position: absolute;
    right: 1rem;
    top: 50%;
    transform: translateY(-50%);
  }
  
  .login-btn.loading .btn-loader {
    display: block;
  }
  
  .demo-info {
    margin-top: 1.5rem;
    text-align: center;
    padding-top: 1.5rem;
    border-top: 1px solid var(--border);
  }
  
  .demo-info p {
    color: var(--text-secondary);
    font-size: 0.9rem;
    margin-bottom: 0.75rem;
  }
  
  .demo-accounts {
    display: flex;
    gap: 0.5rem;
    justify-content: center;
    flex-wrap: wrap;
  }
  
  .demo-account {
    background: var(--surface);
    padding: 0.5rem 0.75rem;
    border-radius: var(--radius);
    font-size: 0.8rem;
    color: var(--text-secondary);
    cursor: pointer;
    transition: all 0.2s ease;
    border: 1px solid var(--border);
  }
  
  .demo-account:hover {
    background: var(--surface-hover);
    color: var(--primary-color);
    border-color: var(--primary-color);
  }
  
  /* App Container */
  .app-container {
    opacity: 0;
    transform: translateY(20px);
    transition: all 0.5s ease;
    min-height: 100vh;
    display: flex;
    flex-direction: column;
  }
  
  .app-container.show {
    opacity: 1;
    transform: translateY(0);
  }
  
  /* Navigation Header */
  .nav-header {
    background: var(--background);
    border-bottom: 1px solid var(--border);
    padding: 1rem 2rem;
    display: flex;
    align-items: center;
    justify-content: space-between;
    position: sticky;
    top: 0;
    z-index: 100;
    backdrop-filter: blur(8px);
  }
  
  .nav-brand h1 {
    font-size: 1.5rem;
    font-weight: 700;
    background: linear-gradient(135deg, var(--primary-color), #8b5cf6);
    -webkit-background-clip: text;
    -webkit-text-fill-color: transparent;
    background-clip: text;
  }
  
  .nav-menu {
    display: flex;
    gap: 0.5rem;
  }
  
  .nav-item {
    padding: 0.75rem 1.5rem;
    background: transparent;
    border: 1px solid transparent;
    border-radius: var(--radius);
    color: var(--text-secondary);
    font-size: 0.9rem;
    font-weight: 500;
    cursor: pointer;
    transition: all 0.2s ease;
  }
  
  .nav-item:hover {
    background: var(--surface);
    color: var(--text-primary);
  }
  
  .nav-item.active {
    background: var(--primary-color);
    color: white;
    border-color: var(--primary-color);
  }
  
  .nav-actions {
    display: flex;
    align-items: center;
    gap: 1rem;
  }
  
  .user-welcome {
    color: var(--text-secondary);
    font-size: 0.9rem;
  }
  
  .theme-toggle {
    background: var(--surface);
    border: 1px solid var(--border);
    border-radius: var(--radius);
    padding: 0.5rem;
    cursor: pointer;
    font-size: 1.1rem;
    transition: all 0.2s ease;
  }
  
  .theme-toggle:hover {
    background: var(--surface-hover);
    border-color: var(--border-hover);
  }
  
  .logout-btn {
    background: var(--error-color);
    color: white;
    border: none;
    border-radius: var(--radius);
    padding: 0.5rem 1rem;
    font-size: 0.9rem;
    font-weight: 500;
    cursor: pointer;
    transition: all 0.2s ease;
  }
  
  .logout-btn:hover {
    background: #dc2626;
    transform: translateY(-1px);
  }
  
  /* Page Content */
  .page-content {
    flex: 1;
    padding: 2rem;
    max-width: 1200px;
    margin: 0 auto;
    width: 100%;
  }
  
  .page-header {
    margin-bottom: 2rem;
    display: flex;
    justify-content: space-between;
    align-items: flex-start;
  }
  
  .page-title-section {
    display: flex;
    align-items: center;
    gap: 1rem;
  }
  
  .back-btn {
    background: var(--surface);
    border: 1px solid var(--border);
    border-radius: var(--radius);
    padding: 0.5rem 1rem;
    color: var(--text-secondary);
    font-size: 0.9rem;
    cursor: pointer;
    transition: all 0.2s ease;
  }
  
  .back-btn:hover {
    background: var(--surface-hover);
    color: var(--text-primary);
  }
  
  .page-header h2 {
    font-size: 2rem;
    font-weight: 700;
    color: var(--text-primary);
    margin-bottom: 0.5rem;
  }
  
  .page-header p {
    color: var(--text-secondary);
    font-size: 1rem;
  }
  
  /* Dashboard Styles */
  .stats-grid {
    display: grid;
    grid-template-columns: repeat(auto-fit, minmax(200px, 1fr));
    gap: 1.5rem;
    margin-bottom: 2rem;
  }
  
  .stat-card {
    background: var(--surface);
    border: 1px solid var(--border);
    border-radius: var(--radius-lg);
    padding: 1.5rem;
    display: flex;
    align-items: center;
    gap: 1rem;
    transition: all 0.2s ease;
  }
  
  .stat-card:hover {
    background: var(--surface-hover);
    border-color: var(--border-hover);
    transform: translateY(-2px);
    box-shadow: var(--shadow);
  }
  
  .stat-icon {
    font-size: 2rem;
    width: 60px;
    height: 60px;
    display: flex;
    align-items: center;
    justify-content: center;
    background: var(--primary-color);
    border-radius: var(--radius);
    color: white;
  }
  
  .stat-content {
    display: flex;
    flex-direction: column;
  }
  
  .stat-number {
    font-size: 2rem;
    font-weight: 700;
    color: var(--text-primary);
    line-height: 1.2;
  }
  
  .stat-label {
    font-size: 0.9rem;
    color: var(--text-secondary);
    font-weight: 500;
  }
  
  .dashboard-section {
    background: var(--surface);
    border: 1px solid var(--border);
    border-radius: var(--radius-lg);
    padding: 1.5rem;
    margin-bottom: 2rem;
  }
  
  .dashboard-section h3 {
    font-size: 1.25rem;
    font-weight: 600;
    color: var(--text-primary);
    margin-bottom: 1rem;
  }
  
  .progress-container {
    margin-bottom: 1rem;
  }
  
  .progress-bar {
    width: 100%;
    height: 12px;
    background: var(--border);
    border-radius: 6px;
    overflow: hidden;
    margin-bottom: 0.5rem;
  }
  
  .progress-fill {
    height: 100%;
    background: linear-gradient(90deg, var(--success-color), #059669);
    border-radius: 6px;
    transition: width 0.5s ease;
    position: relative;
  }
  
  .progress-fill::after {
    content: "";
    position: absolute;
    top: 0;
    left: 0;
    right: 0;
    bottom: 0;
    background: linear-gradient(90deg, transparent, rgba(255, 255, 255, 0.3), transparent);
    animation: shimmer 2s infinite;
  }
  
  @keyframes shimmer {
    0% {
      transform: translateX(-100%);
    }
    100% {
      transform: translateX(100%);
    }
  }
  
  .progress-text {
    text-align: center;
    font-size: 1rem;
    color: var(--text-secondary);
    font-weight: 500;
  }
  
  .recent-activity {
    min-height: 100px;
    display: flex;
    align-items: center;
    justify-content: center;
  }
  
  .no-activity {
    color: var(--text-muted);
    font-style: italic;
  }
  
  .quick-actions {
    display: grid;
    grid-template-columns: repeat(auto-fit, minmax(150px, 1fr));
    gap: 1rem;
  }
  
  .action-card {
    background: var(--background);
    border: 1px solid var(--border);
    border-radius: var(--radius);
    padding: 1.5rem;
    display: flex;
    flex-direction: column;
    align-items: center;
    gap: 0.75rem;
    cursor: pointer;
    transition: all 0.2s ease;
  }
  
  .action-card:hover {
    background: var(--surface-hover);
    border-color: var(--primary-color);
    transform: translateY(-2px);
    box-shadow: var(--shadow);
  }
  
  .action-icon {
    font-size: 2rem;
  }
  
  .action-text {
    font-size: 0.9rem;
    font-weight: 500;
    color: var(--text-primary);
  }
  
  /* Categories Page */
  .categories-grid {
    display: grid;
    grid-template-columns: repeat(auto-fill, minmax(300px, 1fr));
    gap: 1.5rem;
  }
  
  .category-card {
    background: var(--surface);
    border: 1px solid var(--border);
    border-radius: var(--radius-lg);
    padding: 1.5rem;
    cursor: pointer;
    transition: all 0.2s ease;
    animation: fadeInUp 0.5s ease-out;
  }
  
  .category-card:hover {
    background: var(--surface-hover);
    border-color: var(--primary-color);
    transform: translateY(-4px);
    box-shadow: var(--shadow-lg);
  }
  
  .category-header {
    display: flex;
    justify-content: space-between;
    align-items: center;
    margin-bottom: 1rem;
  }
  
  .category-title {
    font-size: 1.25rem;
    font-weight: 600;
    color: var(--text-primary);
  }
  
  .category-count {
    background: var(--primary-color);
    color: white;
    padding: 0.25rem 0.75rem;
    border-radius: 12px;
    font-size: 0.85rem;
    font-weight: 600;
  }
  
  .category-progress {
    margin-bottom: 1rem;
  }
  
  .category-progress-bar {
    width: 100%;
    height: 8px;
    background: var(--border);
    border-radius: 4px;
    overflow: hidden;
    margin-bottom: 0.5rem;
  }
  
  .category-progress-fill {
    height: 100%;
    background: var(--success-color);
    border-radius: 4px;
    transition: width 0.3s ease;
  }
  
  .category-stats {
    display: flex;
    justify-content: space-between;
    font-size: 0.9rem;
    color: var(--text-secondary);
  }
  
  .category-difficulty {
    display: flex;
    gap: 0.5rem;
    margin-top: 1rem;
  }
  
  .difficulty-count {
    padding: 0.25rem 0.5rem;
    border-radius: 8px;
    font-size: 0.8rem;
    font-weight: 500;
  }
  
  .difficulty-count.easy {
    background: #dcfce7;
    color: #166534;
  }
  
  .difficulty-count.medium {
    background: #fef3c7;
    color: #92400e;
  }
  
  .difficulty-count.hard {
    background: #fee2e2;
    color: #991b1b;
  }
  
  [data-theme="dark"] .difficulty-count.easy {
    background: #14532d;
    color: #bbf7d0;
  }
  
  [data-theme="dark"] .difficulty-count.medium {
    background: #451a03;
    color: #fde68a;
  }
  
  [data-theme="dark"] .difficulty-count.hard {
    background: #450a0a;
    color: #fecaca;
  }
  
  /* Problems Page */
  .filters-section {
    background: var(--surface);
    border: 1px solid var(--border);
    border-radius: var(--radius-lg);
    padding: 1.5rem;
    margin-bottom: 2rem;
  }
  
  .search-container {
    position: relative;
    margin-bottom: 1rem;
  }
  
  .search-input {
    width: 100%;
    padding: 0.75rem 1rem 0.75rem 2.5rem;
    border: 2px solid var(--border);
    border-radius: var(--radius);
    font-size: 1rem;
    background: var(--background);
    color: var(--text-primary);
    transition: all 0.2s ease;
  }
  
  .search-input:focus {
    outline: none;
    border-color: var(--primary-color);
    box-shadow: 0 0 0 3px rgb(99 102 241 / 0.1);
  }
  
  .search-icon {
    position: absolute;
    left: 0.75rem;
    top: 50%;
    transform: translateY(-50%);
    font-size: 1.1rem;
    opacity: 0.6;
  }
  
  .clear-search {
    position: absolute;
    right: 0.75rem;
    top: 50%;
    transform: translateY(-50%);
    background: none;
    border: none;
    font-size: 1.2rem;
    color: var(--text-muted);
    cursor: pointer;
    opacity: 0;
    transition: opacity 0.2s ease;
  }
  
  .search-input:not(:placeholder-shown) + .search-icon + .clear-search {
    opacity: 1;
  }
  
  .clear-search:hover {
    color: var(--error-color);
  }
  
  .filters-row {
    display: flex;
    gap: 1rem;
    align-items: center;
    flex-wrap: wrap;
  }
  
  .filter-select {
    padding: 0.5rem 0.75rem;
    border: 1px solid var(--border);
    border-radius: var(--radius);
    background: var(--background);
    color: var(--text-primary);
    font-size: 0.9rem;
    cursor: pointer;
    transition: all 0.2s ease;
    min-width: 120px;
  }
  
  .filter-select:focus {
    outline: none;
    border-color: var(--primary-color);
  }
  
  .difficulty-filter {
    display: flex;
    gap: 0.5rem;
    flex-wrap: wrap;
  }
  
  .difficulty-btn {
    padding: 0.5rem 1rem;
    border: 1px solid var(--border);
    border-radius: var(--radius);
    background: var(--background);
    color: var(--text-secondary);
    font-size: 0.9rem;
    font-weight: 500;
    cursor: pointer;
    transition: all 0.2s ease;
  }
  
  .difficulty-btn:hover {
    background: var(--surface-hover);
    border-color: var(--border-hover);
  }
  
  .difficulty-btn.active {
    background: var(--primary-color);
    color: white;
    border-color: var(--primary-color);
  }
  
  .difficulty-btn.easy.active {
    background: var(--success-color);
    border-color: var(--success-color);
  }
  
  .difficulty-btn.medium.active {
    background: var(--warning-color);
    border-color: var(--warning-color);
  }
  
  .difficulty-btn.hard.active {
    background: var(--error-color);
    border-color: var(--error-color);
  }
  
  /* Problems Container */
  .problems-container {
    display: grid;
    gap: 1rem;
  }
  
  .problem-card {
    background: var(--surface);
    border: 1px solid var(--border);
    border-radius: var(--radius-lg);
    overflow: hidden;
    transition: all 0.3s ease;
    animation: fadeInUp 0.5s ease-out;
  }
  
  @keyframes fadeInUp {
    from {
      opacity: 0;
      transform: translateY(20px);
    }
    to {
      opacity: 1;
      transform: translateY(0);
    }
  }
  
  .problem-card:hover {
    border-color: var(--border-hover);
    box-shadow: var(--shadow);
    transform: translateY(-2px);
  }
  
  .problem-header {
    padding: 1.25rem;
    border-bottom: 1px solid var(--border);
    display: flex;
    justify-content: space-between;
    align-items: flex-start;
    gap: 1rem;
  }
  
  .problem-title-section {
    flex: 1;
  }
  
  .problem-link {
    color: var(--text-primary);
    text-decoration: none;
    font-weight: 600;
    font-size: 1.1rem;
    transition: color 0.2s ease;
  }
  
  .problem-link:hover {
    color: var(--primary-color);
  }
  
  .problem-badges {
    display: flex;
    align-items: center;
    gap: 0.5rem;
    flex-wrap: wrap;
    margin-top: 0.5rem;
  }
  
  .difficulty-badge {
    padding: 0.25rem 0.75rem;
    border-radius: 12px;
    font-size: 0.8rem;
    font-weight: 600;
    text-transform: uppercase;
    letter-spacing: 0.025em;
  }
  
  .difficulty-easy {
    background: #dcfce7;
    color: #166534;
  }
  
  .difficulty-medium {
    background: #fef3c7;
    color: #92400e;
  }
  
  .difficulty-hard {
    background: #fee2e2;
    color: #991b1b;
  }
  
  [data-theme="dark"] .difficulty-easy {
    background: #14532d;
    color: #bbf7d0;
  }
  
  [data-theme="dark"] .difficulty-medium {
    background: #451a03;
    color: #fde68a;
  }
  
  [data-theme="dark"] .difficulty-hard {
    background: #450a0a;
    color: #fecaca;
  }
  
  .status-badge {
    padding: 0.25rem 0.75rem;
    border-radius: 12px;
    font-size: 0.8rem;
    font-weight: 600;
  }
  
  .status-solved {
    background: #dcfce7;
    color: #166534;
  }
  
  .status-unsolved {
    background: #fee2e2;
    color: #991b1b;
  }
  
  [data-theme="dark"] .status-solved {
    background: #14532d;
    color: #bbf7d0;
  }
  
  [data-theme="dark"] .status-unsolved {
    background: #450a0a;
    color: #fecaca;
  }
  
  .problem-tags {
    margin-top: 0.75rem;
    display: flex;
    flex-wrap: wrap;
    gap: 0.25rem;
  }
  
  .tag {
    background: #e0f2fe;
    color: #0277bd;
    padding: 0.125rem 0.5rem;
    border-radius: 8px;
    font-size: 0.75rem;
    font-weight: 500;
  }
  
  [data-theme="dark"] .tag {
    background: #0c4a6e;
    color: #7dd3fc;
  }
  
  .problem-meta {
    padding: 0 1.25rem;
    font-size: 0.9rem;
    color: var(--text-secondary);
    display: flex;
    gap: 1rem;
    flex-wrap: wrap;
  }
  
  .problem-actions {
    padding: 1rem 1.25rem;
    display: flex;
    gap: 0.5rem;
    flex-wrap: wrap;
  }
  
  .solution-btn {
    padding: 0.375rem 0.75rem;
    border: 1px solid var(--border);
    border-radius: var(--radius);
    background: var(--background);
    color: var(--text-secondary);
    font-size: 0.85rem;
    font-weight: 500;
    cursor: pointer;
    transition: all 0.2s ease;
  }
  
  .solution-btn:hover {
    background: var(--surface-hover);
    border-color: var(--border-hover);
    color: var(--text-primary);
  }
  
  .solution-btn.primary {
    background: var(--primary-color);
    color: white;
    border-color: var(--primary-color);
  }
  
  .solution-btn.success {
    background: var(--success-color);
    color: white;
    border-color: var(--success-color);
  }
  
  .solution-btn.warning {
    background: var(--warning-color);
    color: white;
    border-color: var(--warning-color);
  }
  
  .solution-btn.danger {
    background: var(--error-color);
    color: white;
    border-color: var(--error-color);
  }
  
  .solution-content {
    max-height: 0;
    overflow: hidden;
    transition: max-height 0.3s ease;
    background: var(--background);
    border-top: 1px solid var(--border);
  }
  
  .solution-content.show {
    max-height: 1000px;
  }
  
  .solution-code {
    padding: 1.25rem;
    background: #f8fafc;
    font-family: "Monaco", "Menlo", "Ubuntu Mono", monospace;
    font-size: 0.9rem;
    line-height: 1.6;
    white-space: pre-wrap;
    word-wrap: break-word;
    border-radius: var(--radius);
    margin: 1rem;
    border: 1px solid var(--border);
  }
  
  [data-theme="dark"] .solution-code {
    background: #1e293b;
    color: #e2e8f0;
  }
  
  .solution-textarea {
    width: 100%;
    min-height: 200px;
    padding: 1rem;
    border: 1px solid var(--border);
    border-radius: var(--radius);
    font-family: "Monaco", "Menlo", "Ubuntu Mono", monospace;
    font-size: 0.9rem;
    line-height: 1.6;
    background: var(--background);
    color: var(--text-primary);
    resize: vertical;
    margin: 1rem;
  }
  
  .solution-textarea:focus {
    outline: none;
    border-color: var(--primary-color);
    box-shadow: 0 0 0 3px rgb(99 102 241 / 0.1);
  }
  
  .problem-notes {
    margin: 1rem;
    padding: 1rem;
    background: #fff3cd;
    border-radius: var(--radius);
    border-left: 4px solid var(--warning-color);
  }
  
  [data-theme="dark"] .problem-notes {
    background: #451a03;
    color: #fde68a;
  }
  
  /* Empty State */
  .empty-state {
    text-align: center;
    padding: 4rem 2rem;
    color: var(--text-secondary);
  }
  
  .empty-icon {
    font-size: 4rem;
    margin-bottom: 1rem;
    opacity: 0.5;
  }
  
  .empty-state h3 {
    font-size: 1.5rem;
    font-weight: 600;
    margin-bottom: 0.5rem;
    color: var(--text-primary);
  }
  
  .empty-state p {
    font-size: 1rem;
    margin-bottom: 2rem;
    max-width: 400px;
    margin-left: auto;
    margin-right: auto;
  }
  
  /* Buttons */
  .btn {
    padding: 0.5rem 1rem;
    border: 1px solid var(--border);
    border-radius: var(--radius);
    font-size: 0.9rem;
    font-weight: 500;
    cursor: pointer;
    transition: all 0.2s ease;
    display: flex;
    align-items: center;
    gap: 0.5rem;
    text-decoration: none;
  }
  
  .btn-secondary {
    background: var(--background);
    color: var(--text-secondary);
  }
  
  .btn-secondary:hover {
    background: var(--surface-hover);
    border-color: var(--border-hover);
    color: var(--text-primary);
  }
  
  .btn-primary {
    background: var(--primary-color);
    color: white;
    border-color: var(--primary-color);
  }
  
  .btn-primary:hover {
    background: var(--primary-hover);
    transform: translateY(-1px);
    box-shadow: var(--shadow);
  }
  
  .file-input {
    display: none;
  }
  
  /* Modal */
  .modal {
    position: fixed;
    top: 0;
    left: 0;
    width: 100%;
    height: 100%;
    background: rgba(0, 0, 0, 0.5);
    display: flex;
    align-items: center;
    justify-content: center;
    z-index: 1000;
    opacity: 0;
    visibility: hidden;
    transition: all 0.3s ease;
    padding: 1rem;
  }
  
  .modal.show {
    opacity: 1;
    visibility: visible;
  }
  
  .modal-content {
    background: var(--background);
    border-radius: var(--radius-lg);
    box-shadow: var(--shadow-lg);
    width: 100%;
    max-width: 600px;
    max-height: 90vh;
    overflow-y: auto;
    transform: scale(0.9) translateY(20px);
    transition: transform 0.3s ease;
  }
  
  .modal.show .modal-content {
    transform: scale(1) translateY(0);
  }
  
  .modal-header {
    padding: 1.5rem;
    border-bottom: 1px solid var(--border);
    display: flex;
    justify-content: space-between;
    align-items: center;
  }
  
  .modal-header h3 {
    font-size: 1.25rem;
    font-weight: 600;
    color: var(--text-primary);
  }
  
  .close-btn {
    background: none;
    border: none;
    font-size: 1.5rem;
    color: var(--text-muted);
    cursor: pointer;
    padding: 0.25rem;
    border-radius: var(--radius);
    transition: all 0.2s ease;
  }
  
  .close-btn:hover {
    background: var(--surface);
    color: var(--text-primary);
  }
  
  .modal form {
    padding: 1.5rem;
  }
  
  .form-grid {
    display: grid;
    grid-template-columns: 1fr 1fr;
    gap: 1rem;
  }
  
  .form-group {
    margin-bottom: 1.5rem;
  }
  
  .form-group label {
    display: block;
    margin-bottom: 0.5rem;
    font-weight: 500;
    color: var(--text-primary);
    font-size: 0.9rem;
  }
  
  .form-group input,
  .form-group select,
  .form-group textarea {
    width: 100%;
    padding: 0.75rem;
    border: 2px solid var(--border);
    border-radius: var(--radius);
    font-size: 1rem;
    background: var(--background);
    color: var(--text-primary);
    transition: all 0.2s ease;
  }
  
  .form-group input:focus,
  .form-group select:focus,
  .form-group textarea:focus {
    outline: none;
    border-color: var(--primary-color);
    box-shadow: 0 0 0 3px rgb(99 102 241 / 0.1);
  }
  
  .form-group textarea {
    resize: vertical;
    font-family: "Monaco", "Menlo", "Ubuntu Mono", monospace;
    font-size: 0.9rem;
    line-height: 1.6;
  }
  
  .form-actions {
    display: flex;
    gap: 1rem;
    justify-content: flex-end;
    margin-top: 2rem;
    padding-top: 1.5rem;
    border-top: 1px solid var(--border);
  }
  
  /* Goals Modal */
  .goals-content {
    padding: 1.5rem;
  }
  
  .goal-item {
    margin-bottom: 1.5rem;
  }
  
  .goal-item label {
    display: block;
    margin-bottom: 0.5rem;
    font-weight: 500;
    color: var(--text-primary);
  }
  
  .goal-item input {
    width: 100%;
    padding: 0.75rem;
    border: 2px solid var(--border);
    border-radius: var(--radius);
    font-size: 1rem;
    background: var(--background);
    color: var(--text-primary);
    transition: all 0.2s ease;
  }
  
  .goal-item input:focus {
    outline: none;
    border-color: var(--primary-color);
    box-shadow: 0 0 0 3px rgb(99 102 241 / 0.1);
  }
  
  /* Toast Notifications */
  .toast-container {
    position: fixed;
    top: 2rem;
    right: 2rem;
    z-index: 10000;
    display: flex;
    flex-direction: column;
    gap: 0.5rem;
  }
  
  .toast {
    background: var(--background);
    border: 1px solid var(--border);
    border-radius: var(--radius);
    padding: 1rem 1.25rem;
    box-shadow: var(--shadow-lg);
    display: flex;
    align-items: center;
    gap: 0.75rem;
    min-width: 300px;
    animation: slideInRight 0.3s ease-out;
    position: relative;
    overflow: hidden;
  }
  
  @keyframes slideInRight {
    from {
      opacity: 0;
      transform: translateX(100%);
    }
    to {
      opacity: 1;
      transform: translateX(0);
    }
  }
  
  .toast.success {
    border-color: var(--success-color);
  }
  
  .toast.error {
    border-color: var(--error-color);
  }
  
  .toast.warning {
    border-color: var(--warning-color);
  }
  
  .toast-icon {
    font-size: 1.2rem;
  }
  
  .toast-message {
    flex: 1;
    font-size: 0.9rem;
    color: var(--text-primary);
  }
  
  .toast-close {
    background: none;
    border: none;
    color: var(--text-muted);
    cursor: pointer;
    font-size: 1.1rem;
    padding: 0.25rem;
    border-radius: var(--radius);
    transition: all 0.2s ease;
  }
  
  .toast-close:hover {
    background: var(--surface);
    color: var(--text-primary);
  }
  
  .toast-progress {
    position: absolute;
    bottom: 0;
    left: 0;
    height: 3px;
    background: var(--primary-color);
    animation: toastProgress 5s linear forwards;
  }
  
  @keyframes toastProgress {
    from {
      width: 100%;
    }
    to {
      width: 0%;
    }
  }
  
  /* Import Summary Styles */
  .import-summary {
    background: var(--surface);
    border-radius: var(--radius);
    padding: 1rem;
    border: 1px solid var(--border);
  }
  
  .summary-item {
    display: flex;
    justify-content: space-between;
    align-items: center;
    padding: 0.5rem 0;
    border-bottom: 1px solid var(--border);
  }
  
  .summary-item:last-child {
    border-bottom: none;
  }
  
  .summary-label {
    font-weight: 500;
    color: var(--text-primary);
  }
  
  .summary-value {
    font-weight: 600;
    font-size: 1.1rem;
  }
  
  .summary-item.success .summary-value {
    color: var(--success-color);
  }
  
  .summary-item.warning .summary-value {
    color: var(--warning-color);
  }
  
  .summary-item.info .summary-value {
    color: var(--primary-color);
  }
  
  .summary-item.error .summary-value {
    color: var(--error-color);
  }
  
  /* Responsive Design */
  @media (max-width: 768px) {
    .nav-header {
      padding: 1rem;
      flex-direction: column;
      gap: 1rem;
    }
  
    .nav-menu {
      order: 3;
      width: 100%;
      justify-content: center;
    }
  
    .nav-actions {
      order: 2;
      justify-content: space-between;
      width: 100%;
    }
  
    .page-content {
      padding: 1rem;
    }
  
    .page-header {
      flex-direction: column;
      gap: 1rem;
      align-items: stretch;
    }
  
    .stats-grid {
      grid-template-columns: repeat(auto-fit, minmax(150px, 1fr));
      gap: 1rem;
    }
  
    .stat-card {
      padding: 1rem;
    }
  
    .stat-icon {
      width: 50px;
      height: 50px;
      font-size: 1.5rem;
    }
  
    .stat-number {
      font-size: 1.5rem;
    }
  
    .categories-grid {
      grid-template-columns: 1fr;
    }
  
    .filters-row {
      flex-direction: column;
      align-items: stretch;
    }
  
    .difficulty-filter {
      justify-content: center;
    }
  
    .problem-header {
      flex-direction: column;
      align-items: stretch;
      gap: 0.75rem;
    }
  
    .problem-badges {
      justify-content: flex-start;
    }
  
    .problem-actions {
      flex-direction: column;
    }
  
    .solution-btn {
      text-align: center;
    }
  
    .form-grid {
      grid-template-columns: 1fr;
    }
  
    .form-actions {
      flex-direction: column-reverse;
    }
  
    .toast-container {
      top: 1rem;
      right: 1rem;
      left: 1rem;
    }
  
    .toast {
      min-width: auto;
    }
  
    .quick-actions {
      grid-template-columns: repeat(2, 1fr);
    }
  }
  
  @media (max-width: 480px) {
    .nav-menu {
      flex-wrap: wrap;
      gap: 0.25rem;
    }
  
    .nav-item {
      padding: 0.5rem 1rem;
      font-size: 0.8rem;
    }
  
    .stats-grid {
      grid-template-columns: 1fr 1fr;
    }
  
    .quick-actions {
      grid-template-columns: 1fr;
    }
  }
  
  /* Print Styles */
  @media print {
    .nav-header,
    .filters-section,
    .problem-actions,
    .modal,
    .toast-container {
      display: none !important;
    }
  
    .page-content {
      padding: 0;
    }
  
    .problem-card {
      break-inside: avoid;
      margin-bottom: 1rem;
    }
  
    .solution-content.show {
      max-height: none;
    }
  }
  
  /* Accessibility */
  @media (prefers-reduced-motion: reduce) {
    *,
    *::before,
    *::after {
      animation-duration: 0.01ms !important;
      animation-iteration-count: 1 !important;
      transition-duration: 0.01ms !important;
    }
  }
  
  /* Focus styles for keyboard navigation */
  button:focus-visible,
  input:focus-visible,
  select:focus-visible,
  textarea:focus-visible,
  a:focus-visible {
    outline: 2px solid var(--primary-color);
    outline-offset: 2px;
  }
  
  /* High contrast mode support */
  @media (prefers-contrast: high) {
    :root {
      --border: #000000;
      --text-secondary: #000000;
    }
  
    [data-theme="dark"] {
      --border: #ffffff;
      --text-secondary: #ffffff;
    }
  }