Currently in my app when a user types some illegal destination in the URL bar, sees this:

I would like to make it more beautiful and customize it with my own markup and a bit of css.
-Do i need to program that on my app or i have to configure that in the application server?
-Where should i start from if i want to do that?
You can declare custom
<error-page>s inweb.xmldepending on<error-code>(the HTTP status code) or<exception-type>(the full qualified exception class name which was been thrown, e.g.javax.faces.application.ViewExpiredException).You’ve all freedom to fill in the markup in
404.xhtml.