function twentyseventeen_custom_header_setup() {

  add_theme_support(
    'custom-header',
    
/**
 * Filters Twenty Seventeen custom-header support arguments.
 *
 * @since Twenty Seventeen 1.0
 *
 * @param array $args {
 *     An array of custom-header support arguments.
 *
 *     @type string $default-image    Default image of the header.
 *     @type int    $width            Width in pixels of the custom header image. Default 954.
 *     @type int    $height           Height in pixels of the custom header image. Default 1300.
 *     @type string $flex-height      Flex support for height of header.
 *     @type string $video            Video support for header.
 *     @type string $wp-head-callback Callback function used to styles the header image and text
 *                                    displayed on the blog.
 * }
 */
    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',
      )
    )
  );

  register_default_headers(
    array(
      'default-image' => array(
        'url'           => '%s/assets/images/header.jpg',
        'thumbnail_url' => '%s/assets/images/header.jpg',
        'description'   => __( 'Default Header Image', 'twentyseventeen' ),
      ),
    )
  );
}