:root {
    --primary-color: #4361ee;
    --primary-hover: #3a56d4;
    --completed-color: #6c757d;
    --danger-color: #e63946;
    --danger-hover: #d62b39;
    --background-color: #f8f9fa;
    --card-color: #ffffff;
    --text-color: #333333;
    --border-color: #e0e0e0;
    --shadow-color: rgba(0, 0, 0, 0.05);
    --border-radius: 12px;
    --spacing: 16px;
}

* {
    margin: 0;
    padding: 0;
    box-sizing: border-box;
    font-family: 'Nunito', sans-serif;
}

body {
    background-color: var(--background-color);
    color: var(--text-color);
    line-height: 1.6;
    padding: var(--spacing);
    min-height: 100vh;
    display: flex;
    justify-content: center;
    align-items: center;
}

.container {
    width: 100%;
    max-width: 600px;
    margin: 0 auto;
}

.app-header {
    text-align: center;
    margin-bottom: var(--spacing);
}

h1 {
    font-size: 2.5rem;
    font-weight: 700;
    color: var(--primary-color);
    margin-bottom: 0.5rem;
}

h3 {
    font-size: 1.2rem;
    font-weight: 600;
    color: var(--completed-color);
    margin-bottom: var(--spacing);
}

.card {
    background-color: var(--card-color);
    border-radius: var(--border-radius);
    box-shadow: 0 5px 20px var(--shadow-color);
    padding: var(--spacing);
    margin-bottom: var(--spacing);
}

.filter-container {
    margin-bottom: var(--spacing);
}

.btn-group {
    display: flex;
    border-radius: var(--border-radius);
    overflow: hidden;
    box-shadow: 0 2px 10px var(--shadow-color);
}

.btn {
    padding: 10px 20px;
    border: none;
    background-color: var(--card-color);
    cursor: pointer;
    transition: all 0.3s ease;
    font-weight: 600;
    flex: 1;
}

.btn:hover {
    background-color: rgba(67, 97, 238, 0.1);
}

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

.btn-primary {
    background-color: var(--primary-color);
    color: white;
    border-radius: var(--border-radius);
    display: flex;
    align-items: center;
    justify-content: center;
    gap: 6px;
}

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

.todo-list {
    list-style-type: none;
    margin-bottom: var(--spacing);
    max-height: 50vh;
    overflow-y: auto;
}

.todo-item {
    display: flex;
    padding: 12px;
    border-bottom: 1px solid var(--border-color);
    align-items: center;
    animation: fadeIn 0.3s ease;
    transition: background-color 0.3s ease;
}

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

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

.todo-item.done {
    background-color: rgba(108, 117, 125, 0.05);
}

.todo-item-content {
    flex: 1;
    display: flex;
    align-items: center;
}

.todo-checkbox {
    margin-right: 12px;
    width: 18px;
    height: 18px;
    cursor: pointer;
    accent-color: var(--primary-color);
}

.todo-text {
    cursor: pointer;
    word-break: break-word;
    transition: all 0.3s ease;
    padding: 6px 0;
}

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

.edit-todo {
    flex: 1;
    padding: 8px 12px;
    border: 2px solid var(--primary-color);
    border-radius: var(--border-radius);
    font-size: 1rem;
}

.edit-todo:focus {
    outline: none;
}

.delete-btn {
    color: white;
    background-color: var(--danger-color);
    border: none;
    border-radius: 50%;
    width: 36px;
    height: 36px;
    display: flex;
    align-items: center;
    justify-content: center;
    cursor: pointer;
    margin-left: var(--spacing);
    transition: all 0.3s ease;
    font-size: 1rem;
}

.delete-btn:hover {
    background-color: var(--danger-hover);
    transform: scale(1.05);
}

.add-todo-container {
    display: flex;
    margin-top: var(--spacing);
    gap: var(--spacing);
}

.input-todo {
    flex: 1;
    padding: 12px 16px;
    border: 2px solid var(--border-color);
    border-radius: var(--border-radius);
    font-size: 1rem;
    transition: border-color 0.3s ease;
}

.input-todo:focus {
    outline: none;
    border-color: var(--primary-color);
}

.app-footer {
    text-align: center;
    color: var(--completed-color);
    font-size: 0.9rem;
    margin-top: var(--spacing);
}

.empty-state {
    text-align: center;
    color: var(--completed-color);
    padding: 32px 16px;
    border: 2px dashed var(--border-color);
    border-radius: var(--border-radius);
    margin: var(--spacing) 0;
}

/* Responsive adjustments */
@media (max-width: 576px) {
    :root {
        --spacing: 12px;
    }
    
    h1 {
        font-size: 2rem;
    }
    
    .btn {
        padding: 8px 16px;
        font-size: 14px;
    }
    
    .delete-btn {
        width: 44px;
        height: 44px;
    }
}

/* Custom checkbox styling */
input[type="checkbox"] {
    appearance: none;
    -webkit-appearance: none;
    width: 20px;
    height: 20px;
    border: 2px solid var(--primary-color);
    border-radius: 4px;
    outline: none;
    cursor: pointer;
}

input[type="checkbox"]:checked {
    background-color: var(--primary-color);
    position: relative;
}

input[type="checkbox"]:checked::before {
    content: '✓';
    position: absolute;
    color: white;
    font-size: 14px;
    top: 50%;
    left: 50%;
    transform: translate(-50%, -50%);
}