nucleus/base/jinja/404.html (13 lines of code) (raw):
<!DOCTYPE html>
<html LANG="{{ LANG }}" dir="{{ DIR }}">
<head>
<meta content="text/html; charset=UTF-8" http-equiv="Content-Type">
<title>Page not found</title>
</head>
<body>
<h1>Page not found</h1>
<p>
Sorry, but we couldn't find the page you're looking for.
</p>
</body>
</html>