[vbnr]

Hi,
is not necessary to change system files.
Look that code

$wp_html_emails = apply_filters( 'wp_mail_content_type', 'text/plain' ) === 'text/html';

So the right way is just to add filter who returns ‘text/html’

add_filter('wp_mail_content_type', 'text/plain', function () {
    return 'text/html';
});