Cannot make filters work

Hi,
I am trying to manipulate the email templates by using the filter “user_verification_email_templates_data”. I am trying this for example:

add_filter( 'user_verification_email_templates_data', 'custom_user_registered' );

function custom_user_registered( $templates_data )
{
    $templates_data['email_resend_key']['subject'] = "Test";
    return $templates_data;
}

But when I resend the verification mail, everything looks like in the standard settings. What am I missing?

Would it also be possible to use custom email templates in my child theme to overwrite the standard templates? That would be even better than using filters!

Thanks and best regards,
Ben