:root {
    --primary: #10b981;
    --primary-dark: #059669;
    --primary-light: #d1fae5;
    --bg: #f8fafc;
    --card-bg: #ffffff;
    --text-main: #1e293b;
    --text-muted: #64748b;
    --border: #e2e8f0;
    --shadow: 0 4px 6px -1px rgb(0 0 0 / 0.1), 0 2px 4px -2px rgb(0 0 0 / 0.1);
    --shadow-lg: 0 10px 15px -3px rgb(0 0 0 / 0.1), 0 4px 6px -4px rgb(0 0 0 / 0.1);
    --radius: 12px;
    --font: 'Inter', system-ui, -apple-system, sans-serif;
}

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

body {
    font-family: var(--font);
    background-color: var(--bg);
    color: var(--text-main);
    display: flex;
    justify-content: center;
    align-items: flex-start;
    min-height: 100vh;
    padding: 40px 20px;
    line-height: 1.5;
}

.container {
    width: 100%;
    max-width: 500px;
    background: var(--card-bg);
    padding: 32px;
    border-radius: var(--radius);
    box-shadow: var(--shadow-lg);
}

header {
    margin-bottom: 24px;
}

header h1 {
    font-size: 2rem;
    font-weight: 600;
    color: var(--text-main);
}

header p {
    color: var(--text-muted);
    font-size: 0.9rem;
}

/* Input Area */
#todo-form {
    margin-bottom: 24px;
}

.input-group {
    display: flex;
    gap: 12px;
}

#todo-input {
    flex: 1;
    padding: 12px 16px;
    border: 2px solid var(--border);
    border-radius: var(--radius);
    font-family: inherit;
    font-size: 1rem;
    transition: all 0.2s ease;
    outline: none;
}

#todo-input:focus {
    border-color: var(--primary);
    box-shadow: 0 0 0 4px var(--primary-light);
}

#add-btn {
    background: var(--primary);
    color: white;
    border: none;
    border-radius: var(--radius);
    width: 48px;
    height: 48px;
    display: flex;
    align-items: center;
    justify-content: center;
    cursor: pointer;
    transition: background 0.2s ease;
}

#add-btn:hover {
    background: var(--primary-dark);
}

/* Filters */
.filters {
    margin-bottom: 20px;
}

.filter-options {
    display: flex;
    gap: 8px;
}

.filter-btn {
    background: none;
    border: 1px solid var(--border);
    padding: 6px 12px;
    border-radius: 20px;
    font-size: 0.85rem;
    color: var(--text-muted);
    cursor: pointer;
    transition: all 0.2s ease;
}

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

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

/* Todo List */
#todo-list {
    list-style: none;
    margin-bottom: 24px;
}

.todo-item {
    display: flex;
    align-items: center;
    padding: 12px;
    border-bottom: 1px solid var(--border);
    gap: 12px;
    animation: slideIn 0.3s ease forwards;
}

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

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

.todo-item.removing {
    animation: slideOut 0.3s ease forwards;
}

@keyframes slideOut {
    from {
        opacity: 1;
        transform: translateX(0);
    }
    to {
        opacity: 0;
        transform: translateX(20px);
    }
}

/* Custom Checkbox */
.checkbox-container {
    position: relative;
    width: 24px;
    height: 24px;
    cursor: pointer;
}

.checkbox-container input {
    position: absolute;
    opacity: 0;
    cursor: pointer;
    height: 0;
    width: 0;
}

.checkmark {
    position: absolute;
    top: 0;
    left: 0;
    height: 24px;
    width: 24px;
    background-color: transparent;
    border: 2px solid var(--border);
    border-radius: 50%;
    transition: all 0.2s ease;
}

.checkbox-container:hover input ~ .checkmark {
    border-color: var(--primary);
}

.checkbox-container input:checked ~ .checkmark {
    background-color: var(--primary);
    border-color: var(--primary);
}

.checkmark:after {
    content: "";
    position: absolute;
    display: none;
    left: 8px;
    top: 4px;
    width: 5px;
    height: 10px;
    border: solid white;
    border-width: 0 2px 2px 0;
    transform: rotate(45deg);
}

.checkbox-container input:checked ~ .checkmark:after {
    display: block;
    animation: checkAnim 0.2s ease;
}

@keyframes checkAnim {
    from { transform: rotate(45deg) scale(0); }
    to { transform: rotate(45deg) scale(1); }
}

.todo-text {
    flex: 1;
    font-size: 1rem;
    transition: color 0.2s ease;
}

.todo-item.completed .todo-text {
    color: var(--text-muted);
    text-decoration: line-through;
}

.delete-btn {
    background: none;
    border: none;
    color: var(--text-muted);
    cursor: pointer;
    padding: 4px;
    border-radius: 4px;
    opacity: 0;
    transition: all 0.2s ease;
}

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

.delete-btn:hover {
    color: #ef4444;
    background: #fee2e2;
}

/* Empty State */
.empty-state {
    display: none;
    flex-direction: column;
    align-items: center;
    justify-content: center;
    padding: 40px 0;
    text-align: center;
    color: var(--text-muted);
}

.empty-state.visible {
    display: flex;
}

.empty-icon {
    width: 64px;
    height: 64px;
    background: var(--bg);
    border-radius: 50%;
    display: flex;
    align-items: center;
    justify-content: center;
    margin-bottom: 16px;
    color: var(--border);
}

.empty-icon i {
    width: 32px;
    height: 32px;
}

/* Footer */
footer {
    display: flex;
    justify-content: space-between;
    align-items: center;
    padding-top: 24px;
    border-top: 1px solid var(--border);
    font-size: 0.85rem;
    color: var(--text-muted);
}

#clear-completed {
    background: none;
    border: none;
    color: var(--text-muted);
    cursor: pointer;
    transition: color 0.2s ease;
}

#clear-completed:hover {
    color: var(--primary);
}

/* Responsive */
@media (max-width: 480px) {
    body {
        padding: 20px 10px;
    }
    .container {
        padding: 20px;
    }
}