:root {
    --primary: #6366f1;
    --primary-dark: #4f46e5;
    --success: #10b981;
    --warning: #f59e0b;
    --danger: #ef4444;
    --info: #3b82f6;
    
    --bg-primary: #ffffff;
    --bg-secondary: #f9fafb;
    --bg-tertiary: #f3f4f6;
    
    --text-primary: #111827;
    --text-secondary: #6b7280;
    --text-tertiary: #9ca3af;
    
    --border: #e5e7eb;
    --shadow-sm: 0 1px 3px rgba(0, 0, 0, 0.12);
    --shadow-md: 0 4px 6px rgba(0, 0, 0, 0.1);
    --shadow-lg: 0 10px 25px rgba(0, 0, 0, 0.15);
    
    --radius-sm: 6px;
    --radius-md: 10px;
    --radius-lg: 15px;
    
    --transition: all 0.3s cubic-bezier(0.4, 0, 0.2, 1);
}

/* Dark Mode Variables */
body.dark-mode {
    --bg-primary: #1f2937;
    --bg-secondary: #111827;
    --bg-tertiary: #374151;
    
    --text-primary: #f9fafb;
    --text-secondary: #d1d5db;
    --text-tertiary: #9ca3af;
    
    --border: #374151;
}

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

body {
    font-family: 'Segoe UI', -apple-system, BlinkMacSystemFont, sans-serif;
    background: var(--bg-secondary);
    color: var(--text-primary);
    line-height: 1.6;
    transition: background 0.3s ease, color 0.3s ease;
}

.app-container {
    min-height: 100vh;
    display: flex;
    flex-direction: column;
}

/* Header */
.app-header {
    background: var(--bg-primary);
    padding: 1.5rem 2rem;
    box-shadow: var(--shadow-sm);
    display: flex;
    justify-content: space-between;
    align-items: center;
    position: sticky;
    top: 0;
    z-index: 100;
    transition: var(--transition);
}

.header-left {
    display: flex;
    align-items: center;
    gap: 1rem;
}

.header-left i {
    font-size: 2rem;
    color: var(--primary);
}

.header-left h1 {
    font-size: 1.8rem;
    font-weight: 700;
    color: var(--text-primary);
}

.task-count {
    background: var(--bg-tertiary);
    padding: 0.25rem 0.75rem;
    border-radius: var(--radius-sm);
    font-size: 0.9rem;
    color: var(--text-secondary);
    font-weight: 600;
}

.header-right {
    display: flex;
    gap: 0.75rem;
}

.theme-toggle,
.settings-btn {
    width: 45px;
    height: 45px;
    border: none;
    background: var(--bg-tertiary);
    border-radius: var(--radius-md);
    cursor: pointer;
    display: flex;
    align-items: center;
    justify-content: center;
    color: var(--text-secondary);
    transition: var(--transition);
}

.theme-toggle:hover,
.settings-btn:hover {
    background: var(--primary);
    color: white;
    transform: translateY(-2px);
}

/* Main Content */
.app-main {
    display: grid;
    grid-template-columns: 280px 1fr;
    gap: 2rem;
    padding: 2rem;
    flex: 1;
    max-width: 1600px;
    width: 100%;
    margin: 0 auto;
}

/* Sidebar */
.sidebar {
    background: var(--bg-primary);
    border-radius: var(--radius-lg);
    padding: 1.5rem;
    height: fit-content;
    box-shadow: var(--shadow-sm);
    position: sticky;
    top: 100px;
}

.sidebar-section {
    margin-bottom: 2rem;
}

.sidebar-section:last-child {
    margin-bottom: 0;
}

.sidebar-section h3 {
    font-size: 0.85rem;
    text-transform: uppercase;
    letter-spacing: 0.5px;
    color: var(--text-tertiary);
    margin-bottom: 1rem;
    display: flex;
    align-items: center;
    gap: 0.5rem;
}

.filter-list,
.category-list,
.priority-list {
    list-style: none;
}

.filter-item,
.category-item,
.priority-item {
    display: flex;
    align-items: center;
    gap: 0.75rem;
    padding: 0.75rem;
    border-radius: var(--radius-md);
    cursor: pointer;
    transition: var(--transition);
    margin-bottom: 0.5rem;
}

.filter-item:hover,
.category-item:hover,
.priority-item:hover {
    background: var(--bg-tertiary);
}

.filter-item.active,
.category-item.active,
.priority-item.active {
    background: var(--primary);
    color: white;
}

.filter-item i,
.category-item i,
.priority-item i {
    font-size: 1.1rem;
}

.filter-item span:first-of-type,
.category-item span:nth-child(2),
.priority-item span:first-of-type {
    flex: 1;
    font-weight: 500;
}

.count {
    background: var(--bg-tertiary);
    padding: 0.2rem 0.6rem;
    border-radius: var(--radius-sm);
    font-size: 0.85rem;
    font-weight: 600;
}

.filter-item.active .count,
.category-item.active .count,
.priority-item.active .count {
    background: rgba(255, 255, 255, 0.2);
    color: white;
}

.category-color {
    width: 12px;
    height: 12px;
    border-radius: 50%;
    flex-shrink: 0;
}

/* Content Area */
.content-area {
    display: flex;
    flex-direction: column;
    gap: 1.5rem;
}

/* Toolbar */
.toolbar {
    display: flex;
    gap: 1rem;
    align-items: center;
}

.search-box {
    flex: 1;
    position: relative;
}

.search-box i {
    position: absolute;
    left: 1.25rem;
    top: 50%;
    transform: translateY(-50%);
    color: var(--text-tertiary);
}

.search-box input {
    width: 100%;
    padding: 1rem 1rem 1rem 3rem;
    border: 2px solid var(--border);
    border-radius: var(--radius-lg);
    font-size: 1rem;
    background: var(--bg-primary);
    color: var(--text-primary);
    transition: var(--transition);
}

.search-box input:focus {
    outline: none;
    border-color: var(--primary);
    box-shadow: 0 0 0 3px rgba(99, 102, 241, 0.1);
}

.btn-add {
    padding: 1rem 2rem;
    background: var(--primary);
    color: white;
    border: none;
    border-radius: var(--radius-lg);
    font-size: 1rem;
    font-weight: 600;
    cursor: pointer;
    display: flex;
    align-items: center;
    gap: 0.5rem;
    transition: var(--transition);
    white-space: nowrap;
}

.btn-add:hover {
    background: var(--primary-dark);
    transform: translateY(-2px);
    box-shadow: var(--shadow-md);
}

/* Sort Bar */
.sort-bar {
    display: flex;
    justify-content: space-between;
    align-items: center;
    background: var(--bg-primary);
    padding: 1rem 1.5rem;
    border-radius: var(--radius-lg);
    box-shadow: var(--shadow-sm);
}

.sort-options {
    display: flex;
    gap: 0.5rem;
}

.sort-btn {
    padding: 0.6rem 1.25rem;
    background: transparent;
    border: 2px solid var(--border);
    border-radius: var(--radius-md);
    font-size: 0.9rem;
    font-weight: 500;
    color: var(--text-secondary);
    cursor: pointer;
    transition: var(--transition);
    display: flex;
    align-items: center;
    gap: 0.5rem;
}

.sort-btn:hover {
    border-color: var(--primary);
    color: var(--primary);
}

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

.clear-completed {
    padding: 0.6rem 1.25rem;
    background: transparent;
    border: 2px solid var(--danger);
    border-radius: var(--radius-md);
    font-size: 0.9rem;
    font-weight: 500;
    color: var(--danger);
    cursor: pointer;
    transition: var(--transition);
    display: flex;
    align-items: center;
    gap: 0.5rem;
}

.clear-completed:hover {
    background: var(--danger);
    color: white;
}

/* Tasks Container */
.tasks-container {
    display: flex;
    flex-direction: column;
    gap: 1rem;
}

.task-card {
    background: var(--bg-primary);
    border-radius: var(--radius-lg);
    padding: 1.5rem;
    box-shadow: var(--shadow-sm);
    border-left: 4px solid var(--primary);
    transition: var(--transition);
    cursor: move;
}

.task-card:hover {
    box-shadow: var(--shadow-md);
    transform: translateY(-2px);
}

.task-card.completed {
    opacity: 0.6;
}

.task-card.completed .task-title {
    text-decoration: line-through;
}

.task-header {
    display: flex;
    align-items: flex-start;
    gap: 1rem;
    margin-bottom: 0.75rem;
}

.task-checkbox {
    width: 24px;
    height: 24px;
    border: 2px solid var(--border);
    border-radius: var(--radius-sm);
    cursor: pointer;
    display: flex;
    align-items: center;
    justify-content: center;
    transition: var(--transition);
    flex-shrink: 0;
}

.task-checkbox:hover {
    border-color: var(--primary);
}

.task-checkbox.checked {
    background: var(--success);
    border-color: var(--success);
    color: white;
}

.task-content {
    flex: 1;
}

.task-title {
    font-size: 1.1rem;
    font-weight: 600;
    color: var(--text-primary);
    margin-bottom: 0.5rem;
}

.task-description {
    font-size: 0.95rem;
    color: var(--text-secondary);
    line-height: 1.5;
}

.task-actions {
    display: flex;
    gap: 0.5rem;
}

.task-btn {
    width: 36px;
    height: 36px;
    border: none;
    background: var(--bg-tertiary);
    border-radius: var(--radius-md);
    cursor: pointer;
    display: flex;
    align-items: center;
    justify-content: center;
    color: var(--text-secondary);
    transition: var(--transition);
}

.task-btn:hover {
    transform: scale(1.1);
}

.task-btn.edit:hover {
    background: var(--info);
    color: white;
}

.task-btn.delete:hover {
    background: var(--danger);
    color: white;
}

.task-footer {
    display: flex;
    align-items: center;
    gap: 1rem;
    margin-top: 1rem;
    padding-top: 1rem;
    border-top: 1px solid var(--border);
}

.task-category {
    display: flex;
    align-items: center;
    gap: 0.5rem;
    padding: 0.4rem 0.75rem;
    background: var(--bg-tertiary);
    border-radius: var(--radius-sm);
    font-size: 0.85rem;
    font-weight: 500;
}

.task-priority {
    display: flex;
    align-items: center;
    gap: 0.4rem;
    font-size: 0.85rem;
    font-weight: 600;
}

.task-priority.high {
    color: var(--danger);
}

.task-priority.medium {
    color: var(--warning);
}

.task-priority.low {
    color: var(--success);
}

.task-date {
    margin-left: auto;
    display: flex;
    align-items: center;
    gap: 0.5rem;
    font-size: 0.85rem;
    color: var(--text-tertiary);
}

.task-date.overdue {
    color: var(--danger);
    font-weight: 600;
}

/* Empty State */
.empty-state {
    text-align: center;
    padding: 4rem 2rem;
    background: var(--bg-primary);
    border-radius: var(--radius-lg);
    box-shadow: var(--shadow-sm);
}

.empty-state i {
    font-size: 5rem;
    color: var(--text-tertiary);
    margin-bottom: 1.5rem;
}

.empty-state h3 {
    font-size: 1.8rem;
    color: var(--text-primary);
    margin-bottom: 0.5rem;
}

.empty-state p {
    font-size: 1.1rem;
    color: var(--text-secondary);
}

.empty-state.hidden {
    display: none;
}

/* Modal */
.modal {
    position: fixed;
    top: 0;
    left: 0;
    width: 100%;
    height: 100%;
    display: flex;
    align-items: center;
    justify-content: center;
    z-index: 1000;
    opacity: 0;
    pointer-events: none;
    transition: opacity 0.3s ease;
}

.modal.active {
    opacity: 1;
    pointer-events: all;
}

.modal-content {
    background: var(--bg-primary);
    border-radius: var(--radius-lg);
    box-shadow: var(--shadow-lg);
    max-width: 550px;
    width: 100%;
    max-height: 90vh;
    overflow-y: auto;
    transform: scale(0.9);
    transition: transform 0.3s ease;
}

.modal.active .modal-content {
    transform: scale(1);
}

.modal-header {
    display: flex;
    justify-content: space-between;
    align-items: center;
    padding: 1.5rem 2rem;
    border-bottom: 2px solid var(--border);
}

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

.modal-close {
    width: 40px;
    height: 40px;
    border: none;
    background: var(--bg-tertiary);
    border-radius: var(--radius-md);
    cursor: pointer;
    display: flex;
    align-items: center;
    justify-content: center;
    color: var(--text-secondary);
    transition: var(--transition);
}

.modal-close:hover {
    background: var(--danger);
    color: white;
}

#taskForm {
    padding: 2rem;
}

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

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

.form-group input,
.form-group textarea,
.form-group select {
    width: 100%;
    padding: 0.75rem 1rem;
    border: 2px solid var(--border);
    border-radius: var(--radius-md);
    font-size: 1rem;
    font-family: inherit;
    background: var(--bg-secondary);
    color: var(--text-primary);
    transition: var(--transition);
}

.form-group input:focus,
.form-group textarea:focus,
.form-group select:focus {
    outline: none;
    border-color: var(--primary);
    box-shadow: 0 0 0 3px rgba(99, 102, 241, 0.1);
}

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

.form-actions {
    display: flex;
    gap: 1rem;
    justify-content: flex-end;
    margin-top: 2rem;
}

.btn-cancel,
.btn-save {
    padding: 0.75rem 2rem;
    border: none;
    border-radius: var(--radius-md);
    font-size: 1rem;
    font-weight: 600;
    cursor: pointer;
    transition: var(--transition);
    display: flex;
    align-items: center;
    gap: 0.5rem;
}

.btn-cancel {
    background: var(--bg-tertiary);
    color: var(--text-primary);
}

.btn-cancel:hover {
    background: var(--border);
}

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

.btn-save:hover {
    background: var(--primary-dark);
    transform: translateY(-2px);
    box-shadow: var(--shadow-md);
}

/* Overlay */
.overlay {
    position: fixed;
    top: 0;
    left: 0;
    width: 100%;
    height: 100%;
    background: rgba(0, 0, 0, 0.5);
    z-index: 999;
    opacity: 0;
    pointer-events: none;
    transition: opacity 0.3s ease;
    backdrop-filter: blur(4px);
}

.overlay.active {
    opacity: 1;
    pointer-events: all;
}

/* Drag & Drop */
.task-card.dragging {
    opacity: 0.5;
    transform: rotate(2deg);
}

.tasks-container.drag-over {
    background: var(--bg-tertiary);
    border-radius: var(--radius-lg);
}

/* Responsive */
@media (max-width: 1024px) {
    .app-main {
        grid-template-columns: 240px 1fr;
        gap: 1.5rem;
    }
}

@media (max-width: 768px) {
    .app-main {
        grid-template-columns: 1fr;
        padding: 1rem;
    }
    
    .sidebar {
        display: none;
    }
    
    .app-header {
        padding: 1rem;
    }
    
    .header-left h1 {
        font-size: 1.4rem;
    }
    
    .toolbar {
        flex-direction: column;
    }
    
    .btn-add {
        width: 100%;
        justify-content: center;
    }
    
    .sort-bar {
        flex-direction: column;
        gap: 1rem;
    }
    
    .sort-options {
        width: 100%;
        justify-content: center;
    }
    
    .form-row {
        grid-template-columns: 1fr;
    }
}

/* Animations */
@keyframes slideIn {
    from {
        opacity: 0;
        transform: translateY(20px);
    }
    to {
        opacity: 1;
        transform: translateY(0);
    }
}

.task-card {
    animation: slideIn 0.3s ease-out;
}