:root {
      --primary-color: #2563eb;
      --secondary-color: #64748b;
      --success-color: #10b981;
      --warning-color: #f59e0b;
      --danger-color: #ef4444;
      --background-color: #f8fafc;
      --card-bg: #ffffff;
      --text-primary: #1e293b;
      --text-secondary: #64748b;
      --border-color: #e2e8f0;
      --shadow: 0 1px 3px 0 rgba(0, 0, 0, 0.1), 0 1px 2px 0 rgba(0, 0, 0, 0.06);
      --shadow-lg: 0 10px 15px -3px rgba(0, 0, 0, 0.1), 0 4px 6px -2px rgba(0, 0, 0, 0.05);
    }

    body {
      background-color: var(--background-color);
      font-family: 'Inter', -apple-system, BlinkMacSystemFont, sans-serif;
      color: var(--text-primary);
    }

    
    .sidebar {
      background: linear-gradient(135deg, var(--card-bg) 0%, #f1f5f9 100%);
      border-right: 1px solid var(--border-color);
      min-height: 100vh;
      position: sticky;
      top: 0;
    }

    .logo {
      max-height: 60px;
      width: auto;
      transition: transform 0.2s ease;
    }

    .logo:hover {
      transform: scale(1.02);
    }

    .sidebar .nav-link {
      color: var(--text-secondary);
      font-weight: 500;
      padding: 0.75rem 1.5rem;
      margin: 0.25rem 0;
      border-radius: 0.5rem;
      transition: all 0.2s ease;
      position: relative;
    }

    .sidebar .nav-link:hover {
      background-color: rgba(37, 99, 235, 0.1);
      color: var(--primary-color);
      transform: translateX(4px);
    }

   
    .navbar {
      backdrop-filter: blur(10px);
      background-color: rgba(255, 255, 255, 0.95) !important;
    }

   
    .container {
      padding: 2rem 0;
    }

    .invoice-box {
      background: var(--card-bg);
      border-radius: 1rem;
      padding: 2rem;
      margin-bottom: 2rem;
      box-shadow: var(--shadow-lg);
      position: relative;
      overflow: hidden;
    }

    .invoice-box::before {
      content: '';
      position: absolute;
      top: 0;
      left: 0;
      right: 0;
      height: 4px;
      background: linear-gradient(90deg, var(--primary-color), #3b82f6);
    }

    .invoice-box h2 {
      color: var(--text-primary);
      font-weight: 700;
      margin-bottom: 2rem;
      font-size: 2rem;
    }

    
    .header-section {
      display: flex;
      justify-content: space-between;
      align-items: center;
      margin-bottom: 2rem;
      padding: 1.5rem;
      background: linear-gradient(135deg, #f8fafc 0%, #e2e8f0 100%);
      border-radius: 0.75rem;
      border: 1px solid var(--border-color);
    }

    .company-info .logo {
      max-height: 80px;
    }

    .date-section label {
      font-weight: 600;
      color: var(--text-primary);
      margin-bottom: 0.5rem;
    }

    
    .form-label {
      font-weight: 600;
      color: var(--text-primary);
      margin-bottom: 0.5rem;
    }

    .form-control {
      border: 2px solid var(--border-color);
      border-radius: 0.5rem;
      padding: 0.75rem 1rem;
      transition: all 0.2s ease;
      background-color: var(--card-bg);
    }

    .form-control:focus {
      border-color: var(--primary-color);
      box-shadow: 0 0 0 0.2rem rgba(37, 99, 235, 0.25);
      background-color: var(--card-bg);
    }

    .form-control::placeholder {
      color: var(--text-secondary);
    }

    
    .table {
      background: var(--card-bg);
      border-radius: 0.75rem;
      overflow: hidden;
      box-shadow: var(--shadow);
      margin-bottom: 1.5rem;
    }

    .table thead th {
      background: linear-gradient(135deg, #f1f5f9 0%, #e2e8f0 100%);
      border: none;
      color: var(--text-primary);
      font-weight: 600;
      padding: 1rem;
      border-bottom: 2px solid var(--border-color);
    }

    .table tbody td {
      padding: 0.75rem;
      border: none;
      border-bottom: 1px solid var(--border-color);
      vertical-align: middle;
    }

    .table tbody tr:last-child td {
      border-bottom: none;
    }

    .table .form-control {
      border: 1px solid var(--border-color);
      border-radius: 0.375rem;
      padding: 0.5rem 0.75rem;
    }

    .item-row {
      transition: all 0.2s ease;
    }

    .item-row:hover {
      background-color: rgba(37, 99, 235, 0.03);
    }

    
    .btn {
      border-radius: 0.5rem;
      font-weight: 500;
      padding: 0.75rem 1.5rem;
      transition: all 0.2s ease;
      box-shadow: var(--shadow);
      border: none;
      display: inline-flex;
      align-items: center;
      gap: 0.5rem;
    }

    .btn-primary {
      background: linear-gradient(135deg, var(--primary-color), #3b82f6);
      color: white;
    }

    .btn-primary:hover {
      transform: translateY(-1px);
      box-shadow: var(--shadow-lg);
      background: linear-gradient(135deg, #1d4ed8, var(--primary-color));
    }

    .btn-success {
      background: linear-gradient(135deg, var(--success-color), #059669);
      color: white;
    }

    .btn-success:hover {
      transform: translateY(-1px);
      box-shadow: var(--shadow-lg);
      background: linear-gradient(135deg, #059669, #047857);
    }

    .btn-warning {
      background: linear-gradient(135deg, var(--warning-color), #d97706);
      color: white;
    }

    .btn-warning:hover {
      transform: translateY(-1px);
      box-shadow: var(--shadow-lg);
      background: linear-gradient(135deg, #d97706, #b45309);
    }

    .btn-danger {
      background: linear-gradient(135deg, var(--danger-color), #dc2626);
      color: white;
    }

    .btn-danger:hover {
      transform: translateY(-1px);
      box-shadow: var(--shadow-lg);
      background: linear-gradient(135deg, #dc2626, #b91c1c);
    }

    .btn-secondary {
      background: linear-gradient(135deg, var(--secondary-color), #475569);
      color: white;
    }

    .btn-secondary:hover {
      transform: translateY(-1px);
      box-shadow: var(--shadow-lg);
      background: linear-gradient(135deg, #475569, #334155);
    }

    .btn-sm {
      padding: 0.4rem 0.8rem;
      font-size: 0.85rem;
    }

    .remove {
      width: 30px;
      height: 30px;
      border-radius: 50%;
      display: flex;
      align-items: center;
      justify-content: center;
      font-size: 1.2rem;
      line-height: 1;
    }

   
    .form-check {
      margin-bottom: 0.75rem;
    }

    .form-check-input {
      border: 2px solid var(--border-color);
      border-radius: 50%;
    }

    .form-check-input:checked {
      background-color: var(--primary-color);
      border-color: var(--primary-color);
    }

    .form-check-label {
      font-weight: 500;
      color: var(--text-primary);
      margin-left: 0.5rem;
    }

   
    .totals-section {
      background: linear-gradient(135deg, #f8fafc 0%, #e2e8f0 100%);
      padding: 1.5rem;
      border-radius: 0.75rem;
      border: 1px solid var(--border-color);
      margin: 2rem 0;
    }

    .totals-section h5 {
      color: var(--text-primary);
      font-weight: 600;
      margin-bottom: 1rem;
      display: flex;
      align-items: center;
      gap: 0.5rem;
    }

    .total-row {
      padding: 0.5rem 0;
      font-weight: 500;
      border-bottom: 1px solid var(--border-color);
    }

    .total-row:last-child {
      border-bottom: 2px solid var(--primary-color);
      font-weight: 600;
      font-size: 1.1rem;
      color: var(--primary-color);
      margin-top: 0.5rem;
      padding-top: 1rem;
    }

    
    .invoice-preview {
      background: var(--card-bg);
      border-radius: 1rem;
      padding: 2rem;
      box-shadow: var(--shadow-lg);
      border: 1px solid var(--border-color);
      position: relative;
      overflow: hidden;
    }

    .invoice-preview::before {
      content: '';
      position: absolute;
      top: 0;
      left: 0;
      right: 0;
      height: 4px;
      background: linear-gradient(90deg, var(--success-color), #059669);
    }

    .invoice-preview h3 {
      color: var(--text-primary);
      font-weight: 600;
      display: flex;
      align-items: center;
      gap: 0.5rem;
    }

    
    .alert {
      border: none;
      border-radius: 0.75rem;
      padding: 1rem 1.5rem;
      margin-top: 1.5rem;
    }

    .alert-success {
      background: linear-gradient(135deg, rgba(16, 185, 129, 0.1), rgba(5, 150, 105, 0.1));
      color: var(--success-color);
      border-left: 4px solid var(--success-color);
    }

   
    .action-buttons {
      display: flex;
      gap: 1rem;
      flex-wrap: wrap;
      margin-top: 2rem;
      padding-top: 2rem;
      border-top: 2px solid var(--border-color);
    }

   
    @media (max-width: 768px) {
      .container {
        padding: 1rem 0;
      }

      .invoice-box {
        padding: 1.5rem;
        margin: 0 0.5rem 1rem 0.5rem;
      }

      .header-section {
        flex-direction: column;
        gap: 1rem;
        text-align: center;
      }

      .invoice-box h2 {
        font-size: 1.5rem;
      }

      .btn {
        width: 100%;
        justify-content: center;
      }

      .action-buttons {
        flex-direction: column;
      }

      .form-check-inline {
        display: block;
        margin-bottom: 0.5rem;
      }
    }

 
    .fade-in {
      animation: fadeIn 0.5s ease-in;
    }

    @keyframes fadeIn {
      from { opacity: 0; transform: translateY(20px); }
      to { opacity: 1; transform: translateY(0); }
    }

   
    .btn.loading {
      position: relative;
      color: transparent;
    }

    .btn.loading::after {
      content: '';
      position: absolute;
      width: 16px;
      height: 16px;
      top: 50%;
      left: 50%;
      margin-left: -8px;
      margin-top: -8px;
      border: 2px solid #ffffff;
      border-radius: 50%;
      border-top-color: transparent;
      animation: spin 1s linear infinite;
    }

    @keyframes spin {
      to { transform: rotate(360deg); }
    }