/* Reset default browser styles for consistency */
* {
    margin: 0;
    padding: 0;
    box-sizing: border-box;
}

/* Basic body styling */
body {
    font-family: Arial, sans-serif;
    background-color: #f4f4f9;
    color: #333;
    line-height: 1.6;
}

/* Header styling */
header {
    background-color: #4CAF50;
    color: white;
    padding: 1rem;
    text-align: center;
    border-bottom: 2px solid #333;
}

/* Main content styling */
main {
    padding: 2rem;
    text-align: center;
}

/* Button styling with hover effect */
button {
    padding: 0.5rem 1rem;
    font-size: 1rem;
    color: #fff;
    background-color: #007BFF;
    border: none;
    border-radius: 5px;
    cursor: pointer;
    transition: background-color 0.3s ease;
}

button:hover {
    background-color: #0056b3;
}

/* Adjustments for different screen sizes */
@media (max-width: 768px) {
    main {
        padding: 1rem;
    }
}
```

### `app.js`

This JavaScript file enhances interactivity by adding an event listener to a button, allowing actions when the button is clicked:

```javascript
// Ensure DOM is fully loaded before running scripts
document.addEventListener('DOMContentLoaded', function() {
    // Select the button using its ID
    const button = document.getElementById('alertButton');

    // Attach a 'click' event listener to the button
    button.addEventListener('click', function() {
        // Display an alert when the button is clicked
        alert('Button clicked!');
    });
});
```