Wordpress custom php script to match standard page layout

I am writing a custom php script and want to load the theme so it will look like a regular page. I tried the following.

?php
define( 'WP_USE_THEMES', true );
require( $_SERVER['DOCUMENT_ROOT'] . '/wp-load.php' );

$str = '';

get_header();

$str .= 'div id=primary class=content-area';
$str .= 'main id=main class=site-main role=main';

$str .= 'Hello world';

$str .= '/main';
$str .= '/div';

echo $str;

get_footer();
?

It shows the header and footer. However the content appears without borders/margins. A regular page on the site has borders/margins. This one does not.

I could style this page by hand so it matches a regular page. However I am trying to avoid doing it, because I want it to change with the theme.

Any suggestions on how I can get this page styled like a regular page on the site?

Thanks.