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

:root {
    --primary-color: #0D0D0D;
    --secondary-color: #262626;
    --tertiary-color: #595959;
    --quaternary-color: #A6A6A6;
    --background-color: #F2F2F2;
    --success-color: #4CAF50;
    --warning-color: #FF9800;
    --error-color: #f44336;
    --info-color: #2196F3;
    --border-radius: 12px;
    --shadow: 0 2px 10px rgba(0, 0, 0, 0.1);
    --transition: all 0.3s ease;
}

body {
    font-family: -apple-system, BlinkMacSystemFont, 'Segoe UI', Roboto, sans-serif;
    background-color: var(--background-color);
    color: var(--primary-color);
    line-height: 1.6;
    padding-bottom: 80px;
}

/* Header */
.app-header {
    background: linear-gradient(135deg, var(--primary-color), var(--secondary-color));
    color: white;
    padding: 1rem;
    position: sticky;
    top: 0;
    z-index: 100;
    box-shadow: var(--shadow);
}

.app-header h1 {
    font-size: 1.5rem;
    margin-bottom: 0.5rem;
    text-align: center;
}

.header-stats {
    display: flex;
    justify-content: space-around;
    margin-top: 1rem;
}

.stat-item {
    text-align: center;
}

.stat-value {
    display: block;
    font-size: 1.2rem;
    font-weight: bold;
}

.stat-label {
    font-size: 0.8rem;
    opacity: 0.8;
}

/* Main Content */
.app-main {
    padding: 1rem;
    max-width: 1200px;
    margin: 0 auto;
}

.content-section {
    display: none;
    animation: fadeIn 0.3s ease;
}

.content-section.active {
    display: block;
}

@keyframes fadeIn {
    from { opacity: 0; transform: translateY(10px); }
    to { opacity: 1; transform: translateY(0); }
}

/* Section Headers */
.section-header {
    display: flex;
    justify-content: space-between;
    align-items: center;
    margin-bottom: 1.5rem;
    flex-wrap: wrap;
    gap: 1rem;
}

.section-header h2 {
    font-size: 1.5rem;
    color: var(--primary-color);
}

/* Cards */
.dashboard-cards {
    display: grid;
    grid-template-columns: repeat(auto-fit, minmax(280px, 1fr));
    gap: 1rem;
    margin-bottom: 2rem;
}

.card {
    background: white;
    border-radius: var(--border-radius);
    padding: 1.5rem;
    box-shadow: var(--shadow);
    display: flex;
    align-items: center;
    gap: 1rem;
    transition: var(--transition);
}

.card:hover {
    transform: translateY(-2px);
    box-shadow: 0 4px 20px rgba(0, 0, 0, 0.15);
}

.card-icon {
    font-size: 2rem;
    background: var(--background-color);
    width: 60px;
    height: 60px;
    border-radius: 50%;
    display: flex;
    align-items: center;
    justify-content: center;
}

.card-content h3 {
    font-size: 1rem;
    color: var(--tertiary-color);
    margin-bottom: 0.5rem;
}

.card-value {
    font-size: 1.5rem;
    font-weight: bold;
    color: var(--primary-color);
}

/* Buttons */
.btn {
    background: var(--primary-color);
    color: white;
    border: none;
    padding: 0.75rem 1.5rem;
    border-radius: var(--border-radius);
    font-size: 1rem;
    cursor: pointer;
    transition: var(--transition);
    display: inline-flex;
    align-items: center;
    gap: 0.5rem;
    text-decoration: none;
    font-family: inherit;
}

.btn:hover {
    background: var(--secondary-color);
    transform: translateY(-1px);
}

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

.btn-secondary:hover {
    background: var(--tertiary-color);
}

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

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

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

/* Quick Actions */
.quick-actions {
    display: flex;
    gap: 1rem;
    justify-content: center;
    margin-top: 2rem;
    flex-wrap: wrap;
}

/* Recent Activity */
.recent-activity {
    background: white;
    border-radius: var(--border-radius);
    padding: 1.5rem;
    box-shadow: var(--shadow);
    margin-bottom: 2rem;
}

.recent-activity h3 {
    margin-bottom: 1rem;
    color: var(--primary-color);
}

.activity-list {
    max-height: 300px;
    overflow-y: auto;
}

.activity-item {
    padding: 0.75rem 0;
    border-bottom: 1px solid var(--background-color);
    display: flex;
    justify-content: space-between;
    align-items: center;
}

.activity-item:last-child {
    border-bottom: none;
}

.activity-description {
    flex: 1;
}

.activity-time {
    font-size: 0.8rem;
    color: var(--tertiary-color);
}

/* Filters and Search */
.filters, .search-bar {
    display: flex;
    gap: 1rem;
    margin-bottom: 1.5rem;
    flex-wrap: wrap;
}

.filters select, .search-bar input, .period-selector select {
    padding: 0.75rem;
    border: 1px solid var(--quaternary-color);
    border-radius: var(--border-radius);
    font-size: 1rem;
    background: white;
    flex: 1;
    min-width: 200px;
}

/* Lists */
.invoices-list, .clients-list {
    display: grid;
    gap: 1rem;
}

.invoice-item, .client-item {
    background: white;
    border-radius: var(--border-radius);
    padding: 1.5rem;
    box-shadow: var(--shadow);
    transition: var(--transition);
}

.invoice-item:hover, .client-item:hover {
    transform: translateY(-1px);
    box-shadow: 0 4px 20px rgba(0, 0, 0, 0.15);
}

.invoice-header, .client-header {
    display: flex;
    justify-content: space-between;
    align-items: flex-start;
    margin-bottom: 1rem;
    flex-wrap: wrap;
    gap: 1rem;
}

.invoice-info h4, .client-info h4 {
    font-size: 1.2rem;
    margin-bottom: 0.5rem;
}

.invoice-meta, .client-meta {
    font-size: 0.9rem;
    color: var(--tertiary-color);
}

.invoice-status, .client-status {
    padding: 0.25rem 0.75rem;
    border-radius: 20px;
    font-size: 0.8rem;
    font-weight: bold;
    text-transform: uppercase;
}

.status-draft {
    background: #f0f0f0;
    color: var(--tertiary-color);
}

.status-sent {
    background: #e3f2fd;
    color: var(--info-color);
}

.status-paid {
    background: #e8f5e8;
    color: var(--success-color);
}

.status-overdue {
    background: #ffebee;
    color: var(--error-color);
}

.invoice-actions, .client-actions {
    display: flex;
    gap: 0.5rem;
    margin-top: 1rem;
    flex-wrap: wrap;
}

.invoice-actions .btn, .client-actions .btn {
    padding: 0.5rem 1rem;
    font-size: 0.9rem;
}

/* Modals */
.modal {
    display: none;
    position: fixed;
    top: 0;
    left: 0;
    width: 100%;
    height: 100%;
    background: rgba(0, 0, 0, 0.5);
    z-index: 1000;
    padding: 1rem;
    overflow-y: auto;
}

.modal.show {
    display: flex;
    align-items: center;
    justify-content: center;
}

.modal-content {
    background: white;
    border-radius: var(--border-radius);
    width: 100%;
    max-width: 600px;
    max-height: 90vh;
    overflow-y: auto;
    box-shadow: 0 10px 30px rgba(0, 0, 0, 0.3);
}

.modal-header {
    display: flex;
    justify-content: space-between;
    align-items: center;
    padding: 1.5rem;
    border-bottom: 1px solid var(--background-color);
}

.close-btn {
    background: none;
    border: none;
    font-size: 1.5rem;
    cursor: pointer;
    color: var(--tertiary-color);
    padding: 0;
    width: 30px;
    height: 30px;
    display: flex;
    align-items: center;
    justify-content: center;
}

/* Forms */
.invoice-form, .client-form {
    padding: 1.5rem;
}

.form-group {
    margin-bottom: 1.5rem;
}

.form-row {
    display: grid;
    grid-template-columns: 1fr 1fr;
    gap: 1rem;
}

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

.form-group input,
.form-group select,
.form-group textarea {
    width: 100%;
    padding: 0.75rem;
    border: 1px solid var(--quaternary-color);
    border-radius: var(--border-radius);
    font-size: 1rem;
    font-family: inherit;
    transition: var(--transition);
}

.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 rgba(13, 13, 13, 0.1);
}

.form-group textarea {
    resize: vertical;
    min-height: 100px;
}

.form-actions {
    display: flex;
    gap: 1rem;
    justify-content: flex-end;
    padding-top: 1rem;
    border-top: 1px solid var(--background-color);
    flex-wrap: wrap;
}

/* Analytics */
.analytics-grid {
    display: grid;
    grid-template-columns: repeat(auto-fit, minmax(250px, 1fr));
    gap: 1rem;
    margin-bottom: 2rem;
}

.analytics-card {
    background: white;
    border-radius: var(--border-radius);
    padding: 1.5rem;
    box-shadow: var(--shadow);
    text-align: center;
}

.analytics-card h3 {
    font-size: 1rem;
    color: var(--tertiary-color);
    margin-bottom: 1rem;
}

.metric-value {
    font-size: 2rem;
    font-weight: bold;
    color: var(--primary-color);
    margin-bottom: 0.5rem;
}

.metric-change {
    font-size: 0.9rem;
    padding: 0.25rem 0.5rem;
    border-radius: 20px;
}

.metric-change.positive {
    background: #e8f5e8;
    color: var(--success-color);
}

.metric-change.negative {
    background: #ffebee;
    color: var(--error-color);
}

.metric-subtitle {
    font-size: 0.8rem;
    color: var(--tertiary-color);
}

.charts-section {
    display: grid;
    grid-template-columns: repeat(auto-fit, minmax(400px, 1fr));
    gap: 2rem;
    margin-bottom: 2rem;
}

.chart-container {
    background: white;
    border-radius: var(--border-radius);
    padding: 1.5rem;
    box-shadow: var(--shadow);
}

.chart-container h3 {
    margin-bottom: 1rem;
    text-align: center;
}

.chart-container canvas {
    width: 100% !important;
    height: auto !important;
}

.top-clients {
    background: white;
    border-radius: var(--border-radius);
    padding: 1.5rem;
    box-shadow: var(--shadow);
}

.top-clients h3 {
    margin-bottom: 1rem;
}

.top-clients-list {
    display: grid;
    gap: 1rem;
}

.top-client-item {
    display: flex;
    justify-content: space-between;
    align-items: center;
    padding: 1rem;
    background: var(--background-color);
    border-radius: var(--border-radius);
}

.client-rank {
    font-weight: bold;
    color: var(--primary-color);
}

.client-revenue {
    font-weight: bold;
    color: var(--success-color);
}

/* FAQ Section */
.faq-section {
    background: white;
    border-radius: var(--border-radius);
    padding: 2rem;
    box-shadow: var(--shadow);
    margin: 2rem 0;
}

.faq-section h2 {
    text-align: center;
    margin-bottom: 2rem;
    color: var(--primary-color);
}

.faq-container {
    max-width: 800px;
    margin: 0 auto;
}

.faq-item {
    border-bottom: 1px solid var(--background-color);
}

.faq-item:last-child {
    border-bottom: none;
}

.faq-question {
    width: 100%;
    background: none;
    border: none;
    padding: 1.5rem 0;
    text-align: left;
    font-size: 1.1rem;
    font-weight: 600;
    cursor: pointer;
    display: flex;
    justify-content: space-between;
    align-items: center;
    color: var(--primary-color);
    transition: var(--transition);
}

.faq-question:hover {
    color: var(--secondary-color);
}

.faq-icon {
    font-size: 1.2rem;
    transition: var(--transition);
}

.faq-question[aria-expanded="true"] .faq-icon {
    transform: rotate(45deg);
}

.faq-answer {
    padding: 0 0 1.5rem 0;
    color: var(--tertiary-color);
    line-height: 1.6;
    animation: slideDown 0.3s ease;
}

.faq-answer[hidden] {
    display: none;
}

@keyframes slideDown {
    from {
        opacity: 0;
        transform: translateY(-10px);
    }
    to {
        opacity: 1;
        transform: translateY(0);
    }
}

/* Bottom Navigation */
.bottom-nav {
    position: fixed;
    bottom: 0;
    left: 0;
    right: 0;
    background: white;
    border-top: 1px solid var(--quaternary-color);
    display: flex;
    justify-content: space-around;
    padding: 0.5rem 0;
    z-index: 100;
    box-shadow: 0 -2px 10px rgba(0, 0, 0, 0.1);
}

.nav-item {
    display: flex;
    flex-direction: column;
    align-items: center;
    padding: 0.5rem;
    text-decoration: none;
    color: var(--tertiary-color);
    transition: var(--transition);
    border-radius: var(--border-radius);
    min-width: 60px;
}

.nav-item:hover,
.nav-item.active {
    color: var(--primary-color);
    background: var(--background-color);
}

.nav-item i {
    font-size: 1.2rem;
    margin-bottom: 0.25rem;
}

.nav-item span {
    font-size: 0.7rem;
    font-weight: 600;
}

/* Footer */
footer {
    text-align: center;
    padding: 2rem 1rem;
    color: var(--tertiary-color);
    font-size: 0.9rem;
    margin-top: 2rem;
}

/* Empty States */
.empty-state {
    text-align: center;
    color: var(--tertiary-color);
    font-style: italic;
    padding: 2rem;
}

/* Responsive Design */
@media (max-width: 768px) {
    .form-row {
        grid-template-columns: 1fr;
    }
    
    .section-header {
        flex-direction: column;
        align-items: stretch;
    }
    
    .filters {
        flex-direction: column;
    }
    
    .filters select,
    .search-bar input {
        min-width: auto;
    }
    
    .invoice-header,
    .client-header {
        flex-direction: column;
        align-items: stretch;
    }
    
    .invoice-actions,
    .client-actions {
        justify-content: center;
    }
    
    .form-actions {
        justify-content: stretch;
    }
    
    .form-actions .btn {
        flex: 1;
    }
    
    .charts-section {
        grid-template-columns: 1fr;
    }
    
    .chart-container {
        min-width: auto;
    }
    
    .dashboard-cards {
        grid-template-columns: 1fr;
    }
}

@media (max-width: 480px) {
    .app-main {
        padding: 0.5rem;
    }
    
    .modal {
        padding: 0.5rem;
    }
    
    .modal-content {
        max-height: 95vh;
    }
    
    .card {
        padding: 1rem;
    }
    
    .card-icon {
        width: 50px;
        height: 50px;
        font-size: 1.5rem;
    }
    
    .metric-value {
        font-size: 1.5rem;
    }
}

/* Utility Classes */
.text-center { text-align: center; }
.text-right { text-align: right; }
.mb-1 { margin-bottom: 1rem; }
.mb-2 { margin-bottom: 2rem; }
.mt-1 { margin-top: 1rem; }
.mt-2 { margin-top: 2rem; }
.hidden { display: none; }
.visible { display: block; }

/* Loading States */
.loading {
    opacity: 0.6;
    pointer-events: none;
}

.loading::after {
    content: '';
    position: absolute;
    top: 50%;
    left: 50%;
    width: 20px;
    height: 20px;
    margin: -10px 0 0 -10px;
    border: 2px solid var(--quaternary-color);
    border-top: 2px solid var(--primary-color);
    border-radius: 50%;
    animation: spin 1s linear infinite;
}

@keyframes spin {
    0% { transform: rotate(0deg); }
    100% { transform: rotate(360deg); }
}