Source

File: wp-admin/includes/class-custom-image-header.php

public function process_default_headers() {
		global $_wp_default_headers;

		if ( ! isset( $_wp_default_headers ) ) {
			return;
		}

		if ( ! empty( $this->default_headers ) ) {
			return;
		}

		$this->default_headers    = $_wp_default_headers;
		$template_directory_uri   = get_template_directory_uri();
		$stylesheet_directory_uri = get_stylesheet_directory_uri();
		foreach ( array_keys( $this->default_headers ) as $header ) {
			$this->default_headers[ $header ]['url']           = sprintf( $this->default_headers[ $header ]['url'], $template_directory_uri, $stylesheet_directory_uri );
			$this->default_headers[ $header ]['thumbnail_url'] = sprintf( $this->default_headers[ $header ]['thumbnail_url'], $template_directory_uri, $stylesheet_directory_uri );
		}
	}