:root {
    --primary-color: #667eea;
    --primary-hover: #5a6fd8;
    --secondary-color: #764ba2;
    --accent-color: #f093fb;
    
    --success-color: #10b981;
    --warning-color: #f59e0b;
    --danger-color: #ef4444;
    --info-color: #3b82f6;
    
    --bg-primary: #ffffff;
    --bg-secondary: #f8fafc;
    --bg-tertiary: #f1f5f9;
    --bg-modal: rgba(0, 0, 0, 0.5);
    
    --text-primary: #1e293b;
    --text-secondary: #64748b;
    --text-muted: #94a3b8;
    
    --border-color: #e2e8f0;
    --border-hover: #cbd5e1;
    
    --shadow-sm: 0 1px 2px 0 rgba(0, 0, 0, 0.05);
    --shadow-md: 0 4px 6px -1px rgba(0, 0, 0, 0.1), 0 2px 4px -1px 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);
    --shadow-xl: 0 20px 25px -5px rgba(0, 0, 0, 0.1), 0 10px 10px -5px rgba(0, 0, 0, 0.04);
    
    --radius-sm: 0.375rem;
    --radius-md: 0.5rem;
    --radius-lg: 0.75rem;
    --radius-xl: 1rem;
    
    --spacing-xs: 0.25rem;
    --spacing-sm: 0.5rem;
    --spacing-md: 1rem;
    --spacing-lg: 1.5rem;
    --spacing-xl: 2rem;
    --spacing-2xl: 3rem;
    
    --font-size-xs: 0.75rem;
    --font-size-sm: 0.875rem;
    --font-size-base: 1rem;
    --font-size-lg: 1.125rem;
    --font-size-xl: 1.25rem;
    --font-size-2xl: 1.5rem;
    --font-size-3xl: 1.875rem;
    
    --transition: all 0.2s ease-in-out;
    --transition-fast: all 0.15s ease-in-out;
}

[data-theme="dark"] {
    --bg-primary: #0f172a;
    --bg-secondary: #1e293b;
    --bg-tertiary: #334155;
    --bg-modal: rgba(0, 0, 0, 0.8);
    
    --text-primary: #f1f5f9;
    --text-secondary: #cbd5e1;
    --text-muted: #64748b;
    
    --border-color: #334155;
    --border-hover: #475569;
}

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

html {
    font-size: 16px;
    scroll-behavior: smooth;
}

body {
    font-family: 'Inter', -apple-system, BlinkMacSystemFont, 'Segoe UI', system-ui, sans-serif;
    background: var(--bg-secondary);
    color: var(--text-primary);
    line-height: 1.6;
    overflow-x: hidden;
}

button {
    font-family: inherit;
    cursor: pointer;
    border: none;
    outline: none;
    transition: var(--transition);
}

input, textarea, select {
    font-family: inherit;
    outline: none;
    transition: var(--transition);
}

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

.app-header {
    background: var(--bg-primary);
    border-bottom: 1px solid var(--border-color);
    box-shadow: var(--shadow-sm);
    position: sticky;
    top: 0;
    z-index: 100;
}

.header-content {
    max-width: 1400px;
    margin: 0 auto;
    padding: var(--spacing-lg) var(--spacing-xl);
    display: flex;
    align-items: center;
    justify-content: space-between;
}

.app-title {
    font-size: var(--font-size-2xl);
    font-weight: 700;
    color: var(--primary-color);
    display: flex;
    align-items: center;
    gap: var(--spacing-sm);
}

.header-actions {
    display: flex;
    align-items: center;
    gap: var(--spacing-md);
}

.main-content {
    flex: 1;
    max-width: 1400px;
    margin: 0 auto;
    padding: var(--spacing-xl);
    display: grid;
    grid-template-columns: 280px 1fr;
    gap: var(--spacing-2xl);
    align-items: start;
}

.sidebar {
    background: var(--bg-primary);
    border-radius: var(--radius-lg);
    box-shadow: var(--shadow-md);
    padding: var(--spacing-lg);
    position: sticky;
    top: calc(80px + var(--spacing-xl));
}

.sidebar-section {
    margin-bottom: var(--spacing-xl);
}

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

.sidebar-section h3 {
    font-size: var(--font-size-lg);
    font-weight: 600;
    margin-bottom: var(--spacing-md);
    color: var(--text-primary);
}

.filter-options {
    display: flex;
    flex-direction: column;
    gap: var(--spacing-xs);
}

.filter-btn {
    display: flex;
    align-items: center;
    justify-content: space-between;
    padding: var(--spacing-sm) var(--spacing-md);
    background: transparent;
    border: 1px solid transparent;
    border-radius: var(--radius-md);
    color: var(--text-secondary);
    font-size: var(--font-size-sm);
    font-weight: 500;
    text-align: left;
    width: 100%;
}

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

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

.filter-btn i {
    margin-right: var(--spacing-sm);
    width: 16px;
    text-align: center;
}

.count {
    background: rgba(255, 255, 255, 0.2);
    padding: 2px 6px;
    border-radius: var(--radius-sm);
    font-size: var(--font-size-xs);
    font-weight: 600;
}

.filter-btn.active .count {
    background: rgba(255, 255, 255, 0.3);
}

.priority-filters {
    display: flex;
    flex-direction: column;
    gap: var(--spacing-xs);
}

.priority-filter {
    display: flex;
    align-items: center;
    padding: var(--spacing-sm) var(--spacing-md);
    background: transparent;
    border: 1px solid transparent;
    border-radius: var(--radius-md);
    color: var(--text-secondary);
    font-size: var(--font-size-sm);
    font-weight: 500;
    text-align: left;
    width: 100%;
}

.priority-filter:hover {
    background: var(--bg-tertiary);
    color: var(--text-primary);
}

.priority-filter.active {
    background: var(--bg-tertiary);
    color: var(--text-primary);
    border-color: var(--border-hover);
}

.priority-indicator {
    width: 12px;
    height: 12px;
    border-radius: 50%;
    margin-right: var(--spacing-sm);
}

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

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

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

.task-content {
    background: var(--bg-primary);
    border-radius: var(--radius-lg);
    box-shadow: var(--shadow-md);
    overflow: hidden;
}

.content-header {
    padding: var(--spacing-lg);
    border-bottom: 1px solid var(--border-color);
    display: flex;
    align-items: center;
    gap: var(--spacing-lg);
}

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

.search-container i {
    position: absolute;
    left: var(--spacing-md);
    top: 50%;
    transform: translateY(-50%);
    color: var(--text-muted);
}

.search-input {
    width: 100%;
    padding: var(--spacing-sm) var(--spacing-md) var(--spacing-sm) 2.5rem;
    border: 1px solid var(--border-color);
    border-radius: var(--radius-md);
    background: var(--bg-secondary);
    font-size: var(--font-size-sm);
}

.search-input:focus {
    border-color: var(--primary-color);
    box-shadow: 0 0 0 3px rgba(102, 126, 234, 0.1);
}

.sort-select {
    padding: var(--spacing-sm) var(--spacing-md);
    border: 1px solid var(--border-color);
    border-radius: var(--radius-md);
    background: var(--bg-secondary);
    color: var(--text-primary);
    font-size: var(--font-size-sm);
    min-width: 160px;
}

.sort-select:focus {
    border-color: var(--primary-color);
    box-shadow: 0 0 0 3px rgba(102, 126, 234, 0.1);
}

.task-list-container {
    min-height: 400px;
    position: relative;
}

.task-list {
    padding: var(--spacing-lg);
    display: flex;
    flex-direction: column;
    gap: var(--spacing-md);
}

.task-item {
    background: var(--bg-secondary);
    border: 1px solid var(--border-color);
    border-radius: var(--radius-lg);
    padding: var(--spacing-lg);
    transition: var(--transition);
    position: relative;
    overflow: hidden;
}

.task-item:hover {
    border-color: var(--border-hover);
    box-shadow: var(--shadow-md);
    transform: translateY(-1px);
}

.task-item.completed {
    opacity: 0.7;
}

.task-item.overdue {
    border-left: 4px solid var(--danger-color);
}

.task-header {
    display: flex;
    align-items: flex-start;
    justify-content: space-between;
    margin-bottom: var(--spacing-sm);
}

.task-title {
    font-size: var(--font-size-lg);
    font-weight: 600;
    color: var(--text-primary);
    margin-bottom: var(--spacing-xs);
    display: flex;
    align-items: center;
    gap: var(--spacing-sm);
}

.task-title.completed {
    text-decoration: line-through;
    opacity: 0.7;
}

.task-checkbox {
    width: 20px;
    height: 20px;
    border: 2px solid var(--border-color);
    border-radius: var(--radius-sm);
    background: var(--bg-primary);
    display: flex;
    align-items: center;
    justify-content: center;
    cursor: pointer;
    transition: var(--transition);
}

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

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

.task-actions {
    display: flex;
    gap: var(--spacing-xs);
}

.task-description {
    color: var(--text-secondary);
    font-size: var(--font-size-sm);
    margin-bottom: var(--spacing-md);
    line-height: 1.5;
}

.task-meta {
    display: flex;
    align-items: center;
    justify-content: space-between;
    gap: var(--spacing-md);
}

.task-info {
    display: flex;
    align-items: center;
    gap: var(--spacing-lg);
}

.task-due-date {
    display: flex;
    align-items: center;
    gap: var(--spacing-xs);
    font-size: var(--font-size-sm);
    color: var(--text-secondary);
}

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

.task-category {
    padding: var(--spacing-xs) var(--spacing-sm);
    border-radius: var(--radius-sm);
    font-size: var(--font-size-xs);
    font-weight: 500;
    text-transform: uppercase;
    letter-spacing: 0.5px;
}

.task-category.work {
    background: rgba(59, 130, 246, 0.1);
    color: var(--info-color);
}

.task-category.personal {
    background: rgba(16, 185, 129, 0.1);
    color: var(--success-color);
}

.task-category.study {
    background: rgba(168, 85, 247, 0.1);
    color: #a855f7;
}

.task-category.health {
    background: rgba(245, 158, 11, 0.1);
    color: var(--warning-color);
}

.task-category.other {
    background: rgba(107, 114, 128, 0.1);
    color: #6b7280;
}

.task-priority {
    display: flex;
    align-items: center;
    gap: var(--spacing-xs);
}

.empty-state {
    display: flex;
    flex-direction: column;
    align-items: center;
    justify-content: center;
    padding: var(--spacing-2xl);
    text-align: center;
    color: var(--text-muted);
    min-height: 300px;
}

.empty-state i {
    font-size: 4rem;
    margin-bottom: var(--spacing-lg);
    opacity: 0.5;
}

.empty-state h3 {
    font-size: var(--font-size-xl);
    font-weight: 600;
    margin-bottom: var(--spacing-sm);
    color: var(--text-secondary);
}

.empty-state p {
    margin-bottom: var(--spacing-lg);
    font-size: var(--font-size-sm);
}

.btn-primary {
    background: linear-gradient(135deg, var(--primary-color), var(--secondary-color));
    color: white;
    padding: var(--spacing-sm) var(--spacing-lg);
    border-radius: var(--radius-md);
    font-weight: 500;
    font-size: var(--font-size-sm);
    display: flex;
    align-items: center;
    gap: var(--spacing-xs);
    box-shadow: var(--shadow-sm);
}

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

.btn-secondary {
    background: var(--bg-secondary);
    color: var(--text-primary);
    padding: var(--spacing-sm) var(--spacing-lg);
    border: 1px solid var(--border-color);
    border-radius: var(--radius-md);
    font-weight: 500;
    font-size: var(--font-size-sm);
}

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

.btn-danger {
    background: var(--danger-color);
    color: white;
    padding: var(--spacing-sm) var(--spacing-lg);
    border-radius: var(--radius-md);
    font-weight: 500;
    font-size: var(--font-size-sm);
}

.btn-danger:hover {
    background: #dc2626;
    box-shadow: var(--shadow-md);
}

.btn-icon {
    width: 40px;
    height: 40px;
    border-radius: var(--radius-md);
    background: var(--bg-secondary);
    border: 1px solid var(--border-color);
    color: var(--text-secondary);
    display: flex;
    align-items: center;
    justify-content: center;
    font-size: var(--font-size-sm);
}

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

.btn-icon-sm {
    width: 32px;
    height: 32px;
    border-radius: var(--radius-sm);
    background: transparent;
    border: 1px solid transparent;
    color: var(--text-muted);
    display: flex;
    align-items: center;
    justify-content: center;
    font-size: var(--font-size-xs);
}

.btn-icon-sm:hover {
    background: var(--bg-secondary);
    color: var(--text-primary);
    border-color: var(--border-color);
}

.modal {
    position: fixed;
    top: 0;
    left: 0;
    right: 0;
    bottom: 0;
    background: var(--bg-modal);
    display: none;
    align-items: center;
    justify-content: center;
    z-index: 1000;
    backdrop-filter: blur(4px);
}

.modal.show {
    display: flex;
}

.modal-content {
    background: var(--bg-primary);
    border-radius: var(--radius-xl);
    box-shadow: var(--shadow-xl);
    width: 100%;
    max-width: 500px;
    max-height: 90vh;
    overflow-y: auto;
    margin: var(--spacing-lg);
    animation: modalSlideIn 0.3s ease-out;
}

.modal-content.small {
    max-width: 400px;
}

@keyframes modalSlideIn {
    from {
        opacity: 0;
        transform: translateY(-20px) scale(0.95);
    }
    to {
        opacity: 1;
        transform: translateY(0) scale(1);
    }
}

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

.modal-header h2,
.modal-header h3 {
    font-size: var(--font-size-xl);
    font-weight: 600;
    color: var(--text-primary);
}

.btn-close {
    width: 32px;
    height: 32px;
    border-radius: var(--radius-sm);
    background: transparent;
    border: none;
    color: var(--text-muted);
    display: flex;
    align-items: center;
    justify-content: center;
    font-size: var(--font-size-lg);
}

.btn-close:hover {
    background: var(--bg-secondary);
    color: var(--text-primary);
}

.modal-body {
    padding: var(--spacing-xl);
}

.modal-footer {
    padding: var(--spacing-xl);
    border-top: 1px solid var(--border-color);
    display: flex;
    justify-content: flex-end;
    gap: var(--spacing-md);
}

.form-group {
    margin-bottom: var(--spacing-lg);
}

.form-row {
    display: grid;
    grid-template-columns: 1fr 1fr;
    gap: var(--spacing-lg);
}

.form-group label {
    display: block;
    margin-bottom: var(--spacing-xs);
    font-weight: 500;
    color: var(--text-primary);
    font-size: var(--font-size-sm);
}

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

.form-group input:focus,
.form-group textarea:focus,
.form-group select:focus {
    border-color: var(--primary-color);
    box-shadow: 0 0 0 3px rgba(102, 126, 234, 0.1);
}

.form-group textarea {
    resize: vertical;
    min-height: 80px;
}

.toast {
    position: fixed;
    top: 20px;
    right: 20px;
    background: var(--bg-primary);
    border: 1px solid var(--border-color);
    border-radius: var(--radius-md);
    box-shadow: var(--shadow-lg);
    padding: var(--spacing-md);
    z-index: 1100;
    min-width: 300px;
    max-width: 400px;
    animation: slideInRight 0.3s ease-out;
    backdrop-filter: blur(10px);
}

.toast-success { 
    border-left: 4px solid var(--success-color); 
    background: linear-gradient(135deg, var(--bg-primary), rgba(16, 185, 129, 0.05));
}

.toast-error { 
    border-left: 4px solid var(--danger-color); 
    background: linear-gradient(135deg, var(--bg-primary), rgba(239, 68, 68, 0.05));
}

.toast-warning { 
    border-left: 4px solid var(--warning-color); 
    background: linear-gradient(135deg, var(--bg-primary), rgba(245, 158, 11, 0.05));
}

.toast-info { 
    border-left: 4px solid var(--info-color); 
    background: linear-gradient(135deg, var(--bg-primary), rgba(59, 130, 246, 0.05));
}

.toast-content {
    display: flex;
    align-items: center;
    gap: var(--spacing-sm);
}

.toast-content i {
    font-size: var(--font-size-lg);
    flex-shrink: 0;
}

.toast-success .toast-content i { color: var(--success-color); }
.toast-error .toast-content i { color: var(--danger-color); }
.toast-warning .toast-content i { color: var(--warning-color); }
.toast-info .toast-content i { color: var(--info-color); }

.toast-message {
    flex: 1;
    font-size: var(--font-size-sm);
    line-height: 1.4;
    font-weight: 500;
}

.toast-close {
    background: transparent;
    border: none;
    color: var(--text-muted);
    cursor: pointer;
    padding: var(--spacing-xs);
    margin-left: var(--spacing-sm);
    border-radius: var(--radius-sm);
    transition: var(--transition);
    flex-shrink: 0;
}

.toast-close:hover {
    background: var(--bg-secondary);
    color: var(--text-primary);
}

@keyframes slideInRight {
    from { 
        transform: translateX(100%); 
        opacity: 0; 
    }
    to { 
        transform: translateX(0); 
        opacity: 1; 
    }
}

.form-error {
    color: var(--danger-color);
    font-size: var(--font-size-xs);
    margin-top: var(--spacing-xs);
    display: flex;
    align-items: center;
    gap: var(--spacing-xs);
    animation: fadeIn 0.2s ease-in-out;
}

.form-error::before {
    content: "⚠";
    font-size: var(--font-size-sm);
}

.loading {
    opacity: 0.7;
    pointer-events: none;
    position: relative;
}

.loading::after {
    content: "";
    position: absolute;
    top: 50%;
    left: 50%;
    width: 20px;
    height: 20px;
    margin: -10px 0 0 -10px;
    border: 2px solid var(--border-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); }
}

.btn-primary:focus,
.btn-secondary:focus,
.btn-danger:focus,
.btn-icon:focus,
.btn-icon-sm:focus {
    outline: 2px solid var(--primary-color);
    outline-offset: 2px;
}

.search-input:focus,
.sort-select:focus,
.form-group input:focus,
.form-group textarea:focus,
.form-group select:focus {
    outline: none;
    border-color: var(--primary-color);
    box-shadow: 0 0 0 3px rgba(102, 126, 234, 0.1);
}

::selection {
    background: rgba(102, 126, 234, 0.2);
    color: var(--text-primary);
}

.drag-over {
    border: 2px dashed var(--primary-color);
    background: rgba(102, 126, 234, 0.05);
}

.drag-indicator {
    position: fixed;
    top: 50%;
    left: 50%;
    transform: translate(-50%, -50%);
    background: var(--bg-primary);
    border: 2px dashed var(--primary-color);
    border-radius: var(--radius-lg);
    padding: var(--spacing-2xl);
    text-align: center;
    color: var(--primary-color);
    font-size: var(--font-size-lg);
    font-weight: 600;
    z-index: 1000;
    pointer-events: none;
    opacity: 0;
    transition: opacity 0.3s ease-in-out;
}

.drag-indicator.show {
    opacity: 1;
}