Hi dima.08.11,
Hi first of all if you customize any code you need to create a child theme. We recommended not to change the code because if you change the code then while updating to our new version all your customisation code will be lost. So better make child theme and first unhook the functions and the edit the code.
http://codex.wordpress.org/Child_Themes
The file is inside theme folder (interface) -> inc -> structure -> content-extension.php
From this file you can change the 404 page and to change the search form text you can change it from theme folder (interface) -> inc -> structure -> searchform-extension.php
and we are using generic icons for it. to change this code you need language translate. You just need to create a .PO and >MO file and need to add inside the language folder and add your default language inside wp-config.php file.
Then you will be able to change the code.
Thank you!