Can I use CSS to style the default 404 error page provided by my web server?

It depends on the server and its configuration. Some servers allow you to customize the default 404 page with CSS, while others don't. If your server allows it, you'll typically need to place a CSS file in a specific directory (often the same directory as the 404 error HTML file) and link to it from the 404 HTML page. Consult your server's documentation for details.