Generate CSS for Generate Block used outside of the page

generate-blocks-template-part-css.php

<?php

add_filter('generateblocks_do_content', function ($content) {
    $part = 'page-header';
  
    $template_part = get_block_template(get_stylesheet() . '//' . $part, 'wp_template_part');

    if ($extra = $template_part->content) {
        $content = $extra . $content;
    }

    return $content;
});