[Theme Horse Support Team]

Hi levi0322,
Yes this above code must be inside style.css

/*
Theme Name: Clean Retina Child
Theme URI: http://aging-naturally.com/clean-retina-child/
Description: Clean Retina Child Theme
Author: Pauline Downie
Author URI: http://aging-naturally.com
Template: clean-retina
Version: 1.0.0
Tags: light, dark, two-columns, right-sidebar, responsive-layout, accessibility-ready
Text Domain: clean-retina-child
*/

@import url(“../clean-retina/style.css”);

/* =Theme customization starts here
————————————————————– */ 

After doing this this will import all the css from your parent theme

Regardless to page 404

Create one functions.php file
after this paste this code

unhook functions

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

remove_action( 'cleanretina_404_content', 'cleanretina_display_404_page_content', 10 );

}
add_action('init','unhook_thematic_functions');

add_action( 'cleanretina_404_content', 'cleanretina_child_display_404_page_content', 10 );

function cleanretina_child_display_404_page_content() {      ?>   
<div id="content">
		<header class="entry-header">
			<h1 class="entry-title"><?php _e( 'Error 404-Page NOT Found', 'attitude' ); ?></a></h1>
		</header>
		<div class="entry-content clearfix" >
			<p>Thank you</p> 
		</div><!-- .entry-content -->
	</div><!-- #content -->
  <?php 
}

Make sure that you have created the child theme folder inside themes folder

Thank you!