Child theme does not override parent theme values (custom-header)

Question

I have issues with putting my own default custom-header image for my child theme, but am unable to do so without altering the parent theme (twenty seventeen).

I'm able to remove the custom header with:

remove_theme_support('custom-header', 15);

But when I add them back in my child theme, the child theme still shows the twenty seventeen default custom-header image

$custom_header_args = array(
        'default-image'      => get_theme_file_uri( '/assets/images/header.jpg' ),
        'width'              => 2000,
        'height'             => 1200,
        'flex-height'        => true,
        'video'              => true,
    );
    add_theme_support( 'custom-header', $custom_header_args );

This is maybe best demonstrated with the minor modifications in the twenty seventeen /inc/custom-header.php file.

    add_theme_support( 'custom-header', apply_filters( 'twentyseventeen_custom_header_args', array(
        'default-image'      => get_parent_theme_file_uri( '/assets/images/header.jpg' ),
        'width'              => 2000,
        'height'             => 1200,
        'flex-height'        => true,
        'video'              => true,
        'wp-head-callback'   => 'twentyseventeen_header_style',
    ) ) );
    remove_theme_support('custom-header');
    add_theme_support( 'custom-header', apply_filters( 'twentyseventeen_custom_header_args', array(
        'default-image'      => get_theme_file_uri( '/assets/images/header.jpg' ),
        'width'              => 2000,
        'height'             => 1200,
        'flex-height'        => true,
        'video'              => true,
        'wp-head-callback'   => 'twentyseventeen_header_style',
    ) ) );

If I have it like this, the parent theme's header.jpg is shown as the custom header. However, if I comment out the first default-image definition, I see the child theme header.jpg.

So why are the previous values not overwritten? How are they even supposed to be overwritten?

Answer 1

No need to do the remove_theme_support, just put yours in your functions.php file, make sure its a different image (might sound obvious but just in case):

$custom_header_args = array(
    'default-image' => get_theme_file_uri( '/assets/images/header.jpg' ),
    'width' => 2000,
    'height' => 1200,
    'flex-height' => true,
    'video' => true,
);
add_theme_support('custom-header', $custom_header_args);

the child theme custom header theme_support values will always override the values of the parent, its commented in the core:

// Merge in data from previous add_theme_support() calls.
// The first value registered wins. (A child theme is set up first.)
if ( isset( $_wp_theme_features['custom-header'] ) )
    $args[0] = wp_parse_args( $_wp_theme_features['custom-header'][0], $args[0] );

the functions.php of the child theme will run first (registering the theme support first) then the functions.php of the parent.