Theme Horse

Hi Barry,
What you need to do is first create child theme with two file (style.css and functions.php). If you have free theme installed then first remove the free theme and only add interface pro theme.

This is an example how to unhook page 404.php and use your own content.
First of all you need to unhook the functions and then only customize the code in your child theme functions.php. Here is a simple example how to unhook functions and customize the code. Below code should be added inside functions.php of child theme.
Under style.css file add below code
Step 1:

/*
Theme Name: Interface Pro Child
Theme URI: https://www.themehorse.com/themes/interface-pro
Author: Theme Horse
Author URI: https://www.themehorse.com
Description: Interface Pro is a Simple, Clean and Flat Responsive Retina Ready WordPress Theme. It is easily customizable with numerous theme options. Some of the highlighted options are two site layout (Narrow and Wide), 4 layouts for every post/page, 5 Page Templates, 8 widget areas, 6 custom widgets, featured slider to highlight your post/page, 10 social icons, top and bottom info bar to hightlight your contact/email/location and many more. Also has custom background, header, menu, favicon, CSS, webclip icon, and logo. Supports popular plugins like Breadcrumb NavXT, WP-PageNavi and Contact Form 7. It is also translation ready. Get free support at https://www.themehorse.com/support-forum/ and View beautiful demo site at https://www.themehorse.com/preview/interface-pro
Version: 3.0.2
License: GNU General Public License v2 or later
License URI: http://www.gnu.org/licenses/gpl-2.0.html
Tags: white, light, green, two-columns, left-sidebar, right-sidebar, fluid-layout, custom-menu, custom-background, custom-header, featured-images, full-width-template, theme-options, threaded-comments, translation-ready, sticky-post
Text Domain: interface
Template: interface-pro
*/

@import url("../interface-pro/style.css"); 

If you like to add your own css add below this code @import url("../interface/style.css");

Step 2:
Under functions.php file.
First do you like to use google font from our theme or font from outside any source?
If you are using google find from theme typography options then you can select font from theme options and you don’t need to create functions.php file. You can avoid step 2.

If you are using google font that we dont have in theme options panel then do below step too.

under functions.php first you need to unhook the functions. There is already the google font and you need to unhook it and overwrite with your code.

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

remove_action( 'wp_enqueue_scripts', 'interface_scripts_styles_method' );

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

add_action( 'wp_enqueue_scripts', 'interface_scripts_child_styles_method' );

function interface_scripts_child_styles_method() {
	global $interface_theme_setting_value,$interface_default;
   global $array_of_default_settings;
 	$options = wp_parse_args( get_option( 'interface_theme_options', array() ), interface_get_option_defaults()); 
   $options_setting = $interface_theme_setting_value;
   /**
	 * Loads our main stylesheet.
	 */
	// Load our main stylesheet.
	wp_enqueue_style( 'interface_style', get_stylesheet_uri());
	wp_style_add_data( 'interface-ie', 'conditional', 'lt IE 9' ); 
	
	if ('on' == $options['site_design']) {
	//Css for responsive design
	wp_enqueue_style( 'interface-responsive', get_template_directory_uri() . '/css/responsive.css');
	}
	/**
	 * Adds JavaScript to pages with the comment form to support
	 * sites with threaded comments (when in use).
	 */
	if ( is_singular() && comments_open() && get_option( 'thread_comments' ) )
		wp_enqueue_script( 'comment-reply' );

	/**
	 * Register JQuery cycle js file for slider.
	 * Register Jquery fancybox js and css file for fancybox effect.
	 */
	wp_enqueue_script( 'jquery_cycle', INTERFACE_JS_URL . '/jquery.cycle.all.min.js', array( 'jquery' ), '2.9999.5', true );
	wp_register_script( 'jquery_fancybox', INTERFACE_JS_URL . '/jquery.fancybox-1.3.4.pack.js', array('jquery'), '1.3.4', true ); 
 	wp_enqueue_style( 'jquery_fancybox_style', INTERFACE_CSS_URL . '/jquery.fancybox-1.3.4.css', true);  
  $interface_google_font_loads = array();
   if( $interface_default['interface_general_typography'] != $options_setting[ 'interface_general_typography' ] ) :
   	array_push( $interface_google_font_loads,$options_setting[ 'interface_general_typography' ] );
   endif;

    if( $interface_default['interface_navigation'] != $options_setting[ 'interface_navigation' ] ) :
   	array_push( $interface_google_font_loads,$options_setting[ 'interface_navigation' ] );
   endif;

    if( $interface_default['interface_title'] != $options_setting[ 'interface_title' ] ) :
   	array_push( $interface_google_font_loads,$options_setting[ 'interface_title' ] );
   endif;

   if( 'PT Sans' == $options_setting[ 'interface_general_typography' ] || 'PT Sans' == $options_setting[ 'interface_navigation' ] ):
   	array_push( $interface_google_font_loads,'PT+Sans');
  	endif;

  	$interface_google_font_load = implode("|", $interface_google_font_loads);

  	wp_register_style( 'interface_google_font', '//fonts.googleapis.com/css?family='.$interface_google_font_load .':400,700italic,700,400italic');
  	wp_enqueue_style( 'interface_google_font' );
 /**
	 * Enqueue Slider setup js file.
	 * Enqueue Fancy Box setup js and css file.	 
	 */
	 	if( 'revolution-slider' != $options[ 'slider_type' ] && '0' == $options[ 'disable_slider' ] ) { 
		if ( (  'slider-on-everypage' == $options[ 'slider_status' ] ) || ( ( is_home() || is_front_page() ) &&  'slider-on-homepage' == $options[ 'slider_status' ] ) ) {
		wp_enqueue_script( 'interface_slider', INTERFACE_JS_URL . '/interface-slider-setting.js', array( 'jquery_cycle' ), false, true );
		}
	}
	wp_enqueue_script( 'interface_fancybox_setup', INTERFACE_JS_URL . '/interface-custom-fancybox-script.js', array('jquery_fancybox'), false , true);   
	wp_enqueue_script( 'backtotop', INTERFACE_JS_URL. '/backtotop.js', array( 'jquery' ) );
	wp_enqueue_script( 'scripts', INTERFACE_JS_URL. '/scripts.js', array('jquery') );
	wp_enqueue_script( 'jquery_custom', INTERFACE_JS_URL. '/jquery.custom.js', array('jquery') );
	wp_enqueue_script('easing', INTERFACE_JS_URL . '/jquery.easing.1.3.js', array('jquery'), true);
	wp_enqueue_script('quicksand', INTERFACE_JS_URL . '/jquery.quicksand.js', array('jquery'), true);
	wp_enqueue_style( 'jquery_fancybox_style' ); 

} 

Note: Provide your font link here as
wp_register_style( 'interface_google_font', '//fonts.googleapis.com/css?family=Open+Sans');
Thank you!