Hey,
in terms of SEO it is important that 404 page stays in the main design of website (with header and footer). Thus I had to play a bit with the code in Theme 404 page Php file to achieve the desirable result.
@theme developers: please take this idea in account for the future versions.
For those who has confronted with the same problem, I can give you appropriate the code (Design - Edi t- 404 template / 404.php).