Modding Parent Theme functions in Child theme

I've got a theme of which I've created a child theme. Asking conceptually since I don't have this particular Wordpress install in a public place.

The parent theme uses the Customizer to select a color choice (a color-specific css file). I've figured out how/where to add it in the parent theme, but of course, I wouldn't do that.

I'd like to extend/replace the parent theme Customizer choices in the child. It selects a specific css theme to use. I see (as I mentioned) and duplicated how the parent does it, but I can't figure out how to write the function in the child theme to replace that function from the parent theme. I've searched far and wide without luck. I'm not a noob at all and yes, I've scoured the Codex. Any suggestions would be greatly appreciated.

It references two files (in different subdirectories of functions/) headerdata.php and customizer.php. I've tried it by copying each of those (and the directory structure) into the child theme, but, I'm assuming the parent's php is overriding the child php since the parent is loading last.

This is the relevant part in the parent customizer.php. I want to add a pink option in the child theme.

`$wp_customize->add_control('happenstance_css_control', array(
'label' => __('Color Scheme', 'happenstance'),
'section' => 'happenstance_general_settings',
'settings' => 'happenstance_css',
'type' => 'radio',
'choices' => array(
'Blue (default)' => __( 'Blue (default)' , 'happenstance' ),
'Gray' => __( 'Gray' , 'happenstance' ),
'Green' => __( 'Green' , 'happenstance' ),
'Purple' => __( 'Purple' , 'happenstance' ),
),
));`

This is the relevant part in the parent headerdata.php. It's where the css file is actually selected.

'function happenstance_css_include () {
global $happenstance_defaults;
if ( get_theme_mod('happenstance_css', $happenstance_defaults['happenstance_css']) == 'Gray' ){
wp_enqueue_style('happenstance-style-gray', get_template_directory_uri().'/css/colors/gray.css');
}

if ( get_theme_mod('happenstance_css', $happenstance_defaults['happenstance_css']) == 'Green' ){
wp_enqueue_style('happenstance-style-green', get_template_directory_uri().'/css/colors/green.css');
}

if ( get_theme_mod('happenstance_css', $happenstance_defaults['happenstance_css']) == 'Purple' ){
wp_enqueue_style('happenstance-style-purple', get_template_directory_uri().'/css/colors/purple.css');
}

if ( get_theme_mod('happenstance_layout', $happenstance_defaults['happenstance_layout']) == 'Wide' ){
wp_enqueue_style('happenstance-wide-layout', get_template_directory_uri().'/css/wide-layout.css');
}
}
add_action( 'wp_enqueue_scripts', 'happenstance_css_include' );'

(The parent theme is Happenstance https://wordpress.org/themes/happenstance/)

I know what to do in each of those areas and have done it hacking the parent. I need it in the child. It might be as simple "calling" the child directory. I'm not sure and I HATE asking for help, but I'm out of ideas and my sanity is being threatened. Thanks!

The current child theme can be downloaded at https://www.dropbox.com/s/n1qflrv82dct5m8/happenstance-4child.zip . (Mods: Chatted with Predrag last night...we couldn't completely figure it out. We copied the entire parent functions.php to the child, which got the child functions.php recognized, but there are two other files within the directory structure. We couldn't make it work.)

Thanks!