body {
  font-family: Arial, sans-serif;
  margin: 0;
  padding: 20px;
  background-color: #f0f4f8;
}
header {
  text-align: center;
  margin-bottom: 20px;
}
form input, form button {
  margin: 5px;
  padding: 10px;
}
#tasks li {
  margin-bottom: 10px;
  background: #fff;
  padding: 10px;
  border-radius: 5px;
}
```

├── app.js
```js
document.addEventListener("DOMContentLoaded", () => {
  if ('serviceWorker' in navigator) {
    navigator.serviceWorker.register('service-worker.js');
  }
});