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

body {
    font-family: 'Segoe UI', Tahoma, Geneva, Verdana, sans-serif;
    background: linear-gradient(135deg, #667eea 0%, #764ba2 100%);
    min-height: 100vh;
    padding: 20px;
}

header {
    background: white;
    padding: 20px;
    border-radius: 10px;
    margin-bottom: 20px;
    box-shadow: 0 4px 6px rgba(0,0,0,0.1);
}

h1 {
    color: #333;
    margin-bottom: 15px;
}

nav button {
    background: #667eea;
    color: white;
    border: none;
    padding: 10px 20px;
    margin-right: 10px;
    border-radius: 5px;
    cursor: pointer;
    font-size: 14px;
    transition: background 0.3s;
}

nav button:hover {
    background: #5568d3;
}

.container {
    background: white;
    padding: 30px;
    border-radius: 10px;
    box-shadow: 0 4px 6px rgba(0,0,0,0.1);
    max-width: 1200px;
    margin: 0 auto;
}

.form-group {
    margin-bottom: 20px;
}

label {
    display: block;
    margin-bottom: 5px;
    color: #555;
    font-weight: bold;
}

input, textarea, select {
    width: 100%;
    padding: 10px;
    border: 2px solid #ddd;
    border-radius: 5px;
    font-size: 14px;
}

input:focus, textarea:focus, select:focus {
    outline: none;
    border-color: #667eea;
}

.btn-primary {
    background: #667eea;
    color: white;
    border: none;
    padding: 12px 30px;
    border-radius: 5px;
    cursor: pointer;
    font-size: 16px;
    transition: background 0.3s;
}

.btn-primary:hover {
    background: #5568d3;
}

.btn-success {
    background: #28a745;
    color: white;
}

.btn-success:hover {
    background: #218838;
}

.btn-danger {
    background: #dc3545;
    color: white;
}

.btn-danger:hover {
    background: #c82333;
}

.products-grid {
    display: grid;
    grid-template-columns: repeat(auto-fill, minmax(280px, 1fr));
    gap: 20px;
    margin-top: 20px;
}

.product-card {
    background: #f8f9fa;
    padding: 20px;
    border-radius: 10px;
    border: 2px solid #ddd;
    transition: transform 0.3s, box-shadow 0.3s;
}

.product-card:hover {
    transform: translateY(-5px);
    box-shadow: 0 6px 12px rgba(0,0,0,0.15);
}

.product-card h3 {
    color: #333;
    margin-bottom: 10px;
}

.product-card .price {
    color: #667eea;
    font-size: 20px;
    font-weight: bold;
    margin: 10px 0;
}

.product-card .description {
    color: #666;
    margin-bottom: 15px;
    font-size: 14px;
}

.invoice {
    background: #f8f9fa;
    padding: 30px;
    border-radius: 10px;
    border: 2px dashed #667eea;
    margin-top: 20px;
}

.invoice h2 {
    color: #333;
    margin-bottom: 20px;
    text-align: center;
}

.invoice-info {
    background: white;
    padding: 20px;
    border-radius: 5px;
    margin-bottom: 20px;
}

.invoice-info p {
    margin: 10px 0;
    color: #555;
}

.invoice-table {
    width: 100%;
    border-collapse: collapse;
    background: white;
    margin-top: 20px;
}

.invoice-table th,
.invoice-table td {
    padding: 12px;
    text-align: left;
    border-bottom: 1px solid #ddd;
}

.invoice-table th {
    background: #667eea;
    color: white;
}

.invoice-table tr:hover {
    background: #f8f9fa;
}

.total-price {
    text-align: right;
    font-size: 24px;
    color: #667eea;
    font-weight: bold;
    margin-top: 20px;
}

.btn-group {
    display: flex;
    gap: 10px;
    margin-top: 20px;
}

.hidden {
    display: none;
}

.alert {
    padding: 15px;
    border-radius: 5px;
    margin-bottom: 20px;
}

.alert-success {
    background: #d4edda;
    color: #155724;
    border: 1px solid #c3e6cb;
}

.alert-danger {
    background: #f8d7da;
    color: #721c24;
    border: 1px solid #f5c6cb;
}

.back-btn {
    background: #6c757d;
    color: white;
    padding: 10px 20px;
    border: none;
    border-radius: 5px;
    cursor: pointer;
    margin-bottom: 20px;
}

.back-btn:hover {
    background: #5a6268;
}