I added to .htaccess the line to the 404 document on mantra's folder.
And when i got a 404 page it appears this message:
Fatal error: Call to undefined function get_header() in /home/blavla/public_html/wp-content/themes/mantra/404.php on line 10
To install the theme i searched on wordpress theme search included in wordpress and installed it and activated.
HELP ME Plz