_flatten_blocks example

function _inject_theme_attribute_in_block_template_content( $template_content ) {
    $has_updated_content = false;
    $new_content         = '';
    $template_blocks     = parse_blocks( $template_content );

    $blocks = _flatten_blocks( $template_blocks );
    foreach ( $blocks as &$block ) {
        if (
            'core/template-part' === $block['blockName'] &&
            ! isset( $block['attrs']['theme'] )
        ) {
            $block['attrs']['theme'] = get_stylesheet();
            $has_updated_content     = true;
        }
    }

    if ( $has_updated_content ) {