[Theme Horse Support Team]

Hi maria.economides,
Yes upto there you are correct
To import your css use it

@import url(“../interface-pro/style.css”);
/* =Theme customization starts here
————————————————————– */
[rest of custom style css]

and create a functions.php if you want to override file then,
For example to unhook the functions and over ride the parent theme for page 404.php
unhook functions

// Unhook default Thematic functions
function unhook_thematic_functions() {
    // Don't forget the position number if the original function has one

remove_action( 'interface_404_content', 'interface_display_404_page_content', 10 );

}
add_action('init','unhook_thematic_functions'); // removes the header content by using hook interface_header

add_action( 'interface_404_content', 'interface_child_display_404_page_content', 10 );
/**
 * function to show the footer info, copyright information
 */
 
 
function interface_child_display_404_page_content() {      ?>   
<div id="content">
		<header class="entry-header">
			<h1 class="entry-title"><?php _e( 'Error 404-Page NOT Found', 'interface' ); ?></h1>
		</header>
		<div class="entry-content clearfix" >
			<p>Thank you</p> 
		</div><!-- .entry-content -->
	</div><!-- #content -->
  <?php 
}

For more information http://codex.wordpress.org/Child_Themes

Thank You!