:root {
    --primary-color: #0052CC;
    --primary-hover: #0747A6;
    --secondary-color: #6554C0;
    --success-color: #36B37E;
    --warning-color: #FFAB00;
    --danger-color: #DE350B;
    --background: #FFFFFF;
    --surface: #F4F5F7;
    --text-primary: #172B4D;
    --text-secondary: #6B778C;
    --border-color: #DFE1E6;
    --shadow-sm: 0 1px 2px rgba(0, 0, 0, 0.08);
    --shadow-md: 0 4px 8px rgba(0, 0, 0, 0.12);
    --shadow-lg: 0 8px 16px rgba(0, 0, 0, 0.15);
    --radius: 8px;
    --transition: all 0.3s cubic-bezier(0.4, 0, 0.2, 1);
    --progress: 0%;
}

body.dark-mode {
    --background: #0D1117;
    --surface: #161B22;
    --text-primary: #F0F6FC;
    --text-secondary: #8B949E;
    --border-color: #30363D;
    --primary-color: #1F6FEB;
    --primary-hover: #388BFD;
}

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

body {
    font-family: -apple-system, BlinkMacSystemFont, 'Segoe UI', 'Roboto', 'Oxygen', 'Ubuntu', 'Fira Sans', 'Droid Sans', 'Helvetica Neue', sans-serif;
    background: var(--background);
    color: var(--text-primary);
    line-height: 1.6;
    transition: var(--transition);
}

.container {
    max-width: 1200px;
    margin: 0 auto;
    padding: 20px;
}

header {
    display: flex;
    justify-content: space-between;
    align-items: center;
    padding: 20px 0;
    border-bottom: 1px solid var(--border-color);
    margin-bottom: 40px;
}

h1 {
    font-size: 28px;
    font-weight: 600;
    background: linear-gradient(135deg, var(--primary-color), var(--secondary-color));
    -webkit-background-clip: text;
    -webkit-text-fill-color: transparent;
    background-clip: text;
}

.header-controls {
    display: flex;
    gap: 12px;
    align-items: center;
}

#language-selector {
    padding: 8px 12px;
    border: 1px solid var(--border-color);
    border-radius: var(--radius);
    background: var(--surface);
    color: var(--text-primary);
    font-size: 14px;
    cursor: pointer;
    transition: var(--transition);
}

#language-selector:hover {
    border-color: var(--primary-color);
}

#theme-toggle {
    width: 40px;
    height: 40px;
    border: 1px solid var(--border-color);
    border-radius: var(--radius);
    background: var(--surface);
    font-size: 20px;
    cursor: pointer;
    transition: var(--transition);
}

#theme-toggle:hover {
    background: var(--primary-color);
    border-color: var(--primary-color);
    transform: rotate(20deg);
}

main {
    display: grid;
    gap: 40px;
}

.timer-section {
    background: var(--surface);
    border-radius: var(--radius);
    padding: 40px;
    box-shadow: var(--shadow-md);
    position: relative;
    overflow: hidden;
}

.timer-section::before {
    content: '';
    position: absolute;
    top: 0;
    left: 0;
    width: var(--progress);
    height: 4px;
    background: linear-gradient(90deg, var(--primary-color), var(--secondary-color));
    transition: width 1s linear;
}

.timer-display {
    text-align: center;
    margin-bottom: 30px;
}

.timer {
    font-size: 72px;
    font-weight: 300;
    font-variant-numeric: tabular-nums;
    letter-spacing: -2px;
    color: var(--primary-color);
    text-shadow: 0 4px 12px rgba(0, 82, 204, 0.2);
}

.session-type {
    font-size: 18px;
    color: var(--text-secondary);
    margin-top: 10px;
    text-transform: uppercase;
    letter-spacing: 1px;
}

.timer-controls {
    display: flex;
    justify-content: center;
    gap: 16px;
    margin-bottom: 30px;
}

.btn {
    padding: 12px 32px;
    border: none;
    border-radius: var(--radius);
    font-size: 16px;
    font-weight: 500;
    cursor: pointer;
    transition: var(--transition);
    text-transform: uppercase;
    letter-spacing: 0.5px;
}

.btn:disabled {
    opacity: 0.5;
    cursor: not-allowed;
}

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

.btn-primary:hover:not(:disabled) {
    background: var(--primary-hover);
    transform: translateY(-2px);
    box-shadow: var(--shadow-lg);
}

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

.btn-secondary:hover:not(:disabled) {
    background: #FF991F;
    transform: translateY(-2px);
    box-shadow: var(--shadow-lg);
}

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

.btn-tertiary:hover:not(:disabled) {
    background: var(--text-secondary);
    color: white;
    transform: translateY(-2px);
}

.session-settings {
    display: grid;
    grid-template-columns: repeat(auto-fit, minmax(150px, 1fr));
    gap: 20px;
}

.setting-group {
    display: flex;
    flex-direction: column;
    gap: 8px;
}

.setting-group label {
    font-size: 14px;
    color: var(--text-secondary);
    font-weight: 500;
}

.setting-group input {
    padding: 10px;
    border: 1px solid var(--border-color);
    border-radius: var(--radius);
    background: var(--background);
    color: var(--text-primary);
    font-size: 16px;
    transition: var(--transition);
}

.setting-group input:focus {
    outline: none;
    border-color: var(--primary-color);
    box-shadow: 0 0 0 3px rgba(0, 82, 204, 0.1);
}

.progress-section {
    background: var(--surface);
    border-radius: var(--radius);
    padding: 30px;
    box-shadow: var(--shadow-md);
}

.tabs {
    display: flex;
    gap: 8px;
    margin-bottom: 30px;
    border-bottom: 2px solid var(--border-color);
}

.tab-btn {
    padding: 12px 24px;
    background: none;
    border: none;
    color: var(--text-secondary);
    font-size: 15px;
    font-weight: 500;
    cursor: pointer;
    position: relative;
    transition: var(--transition);
}

.tab-btn:hover {
    color: var(--text-primary);
}

.tab-btn.active {
    color: var(--primary-color);
}

.tab-btn.active::after {
    content: '';
    position: absolute;
    bottom: -2px;
    left: 0;
    right: 0;
    height: 2px;
    background: var(--primary-color);
}

.tab-panel {
    display: none;
}

.tab-panel.active {
    display: block;
    animation: fadeIn 0.3s ease-in-out;
}

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

.stats-grid {
    display: grid;
    grid-template-columns: repeat(auto-fit, minmax(200px, 1fr));
    gap: 20px;
    margin-bottom: 30px;
}

.stat-card {
    background: var(--background);
    padding: 20px;
    border-radius: var(--radius);
    border: 1px solid var(--border-color);
    transition: var(--transition);
}

.stat-card:hover {
    transform: translateY(-4px);
    box-shadow: var(--shadow-lg);
    border-color: var(--primary-color);
}

.stat-value {
    font-size: 32px;
    font-weight: 600;
    color: var(--primary-color);
    margin-bottom: 8px;
}

.stat-label {
    font-size: 14px;
    color: var(--text-secondary);
    text-transform: uppercase;
    letter-spacing: 0.5px;
}

canvas {
    max-width: 100%;
    height: auto;
    margin-top: 20px;
}

.task-section {
    background: var(--surface);
    border-radius: var(--radius);
    padding: 30px;
    box-shadow: var(--shadow-md);
}

.task-section h3 {
    margin-bottom: 20px;
    color: var(--text-primary);
}

.task-input-group {
    display: flex;
    gap: 12px;
    margin-bottom: 20px;
}

#task-input {
    flex: 1;
    padding: 12px;
    border: 1px solid var(--border-color);
    border-radius: var(--radius);
    background: var(--background);
    color: var(--text-primary);
    font-size: 15px;
    transition: var(--transition);
}

#task-input:focus {
    outline: none;
    border-color: var(--primary-color);
    box-shadow: 0 0 0 3px rgba(0, 82, 204, 0.1);
}

.btn-small {
    padding: 12px 24px;
    font-size: 14px;
}

.task-list {
    display: flex;
    flex-direction: column;
    gap: 12px;
}

.task-item {
    display: flex;
    align-items: center;
    gap: 12px;
    padding: 12px;
    background: var(--background);
    border: 1px solid var(--border-color);
    border-radius: var(--radius);
    transition: var(--transition);
}

.task-item:hover {
    border-color: var(--primary-color);
    box-shadow: var(--shadow-sm);
}

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

.task-item.completed span {
    text-decoration: line-through;
    color: var(--text-secondary);
}

.task-item input[type="checkbox"] {
    width: 20px;
    height: 20px;
    cursor: pointer;
}

.task-item span {
    flex: 1;
    color: var(--text-primary);
}

.delete-btn {
    width: 28px;
    height: 28px;
    border: none;
    background: var(--danger-color);
    color: white;
    border-radius: 50%;
    font-size: 18px;
    cursor: pointer;
    transition: var(--transition);
    opacity: 0;
}

.task-item:hover .delete-btn {
    opacity: 1;
}

.delete-btn:hover {
    background: #BF2600;
    transform: scale(1.1);
}

.achievement-section {
    margin-top: 30px;
}

.achievement-section h4 {
    margin-bottom: 20px;
    color: var(--text-primary);
}

.achievements-grid {
    display: grid;
    grid-template-columns: repeat(auto-fill, minmax(100px, 1fr));
    gap: 16px;
}

.achievement {
    display: flex;
    flex-direction: column;
    align-items: center;
    gap: 8px;
    padding: 16px;
    background: var(--background);
    border: 2px solid var(--border-color);
    border-radius: var(--radius);
    transition: var(--transition);
}

.achievement.earned {
    border-color: var(--success-color);
    background: linear-gradient(135deg, rgba(54, 179, 126, 0.1), rgba(54, 179, 126, 0.05));
}

.achievement.locked {
    opacity: 0.5;
    filter: grayscale(1);
}

.achievement-icon {
    font-size: 32px;
}

.achievement-name {
    font-size: 12px;
    text-align: center;
    color: var(--text-secondary);
}

.achievement-notification {
    position: fixed;
    top: 20px;
    right: 20px;
    padding: 16px 24px;
    background: var(--success-color);
    color: white;
    border-radius: var(--radius);
    box-shadow: var(--shadow-lg);
    display: flex;
    align-items: center;
    gap: 12px;
    transform: translateX(400px);
    transition: var(--transition);
    z-index: 1000;
}

.achievement-notification.show {
    transform: translateX(0);
}

footer {
    margin-top: 60px;
    padding: 30px 0;
    border-top: 1px solid var(--border-color);
    text-align: center;
}

footer p {
    color: var(--text-secondary);
    font-size: 14px;
}

@media (max-width: 768px) {
    .container {
        padding: 16px;
    }
    
    header {
        flex-direction: column;
        gap: 20px;
    }
    
    h1 {
        font-size: 24px;
    }
    
    .timer {
        font-size: 48px;
    }
    
    .timer-section {
        padding: 24px;
    }
    
    .timer-controls {
        flex-direction: column;
    }
    
    .btn {
        width: 100%;
    }
    
    .tabs {
        overflow-x: auto;
        -webkit-overflow-scrolling: touch;
    }
    
    .tab-btn {
        white-space: nowrap;
    }
    
    .stats-grid {
        grid-template-columns: 1fr 1fr;
    }
    
    .task-input-group {
        flex-direction: column;
    }
    
    .achievements-grid {
        grid-template-columns: repeat(3, 1fr);
    }
}

@media (prefers-reduced-motion: reduce) {
    * {
        animation: none !important;
        transition: none !important;
    }
}