正如您现在所述，主题A和主题B都已经是子主题，也许您可以尝试将其添加到funcions.php 主题B中的文件-

add_action(\'wp_enqueue_scripts\', \'enqueue_front_scripts\');
function enqueue_front_scripts(){

    /** Switch to the parent blog */
    switch_to_blog(1);  // You may need to change the ID, I don\'t know what ID you main site has
    
    /** Grab the path of the parents \'header.php\' file */
    $main_header_style_path =  get_stylesheet_directory_uri() . \'/header.css\';

    /** Restore the current blog */
    restore_current_blog();
    
    /** Enqueue the main header styling */
    wp_enqueue_style(\'main-header\', $main_header_style_path);
    
}

function get_main_header(){

    /** Switch to the parent blog */
    switch_to_blog(1);  // You may need to change the ID, I don\'t know what ID you main site has

    /** Grab the path of the parents \'header.php\' file */
    $main_header_path =  get_stylesheet_directory_uri() . \'/header.php\';

    /** Output the main header */
    require_once($main_header_path);

    /** Restore the current blog */
    restore_current_blog(); // Don\'t restore until after you have included the header, otherwise you \'get_blogino()\', etc. calls will reference Theme B
    
}

然后要包含这两个头文件，您可以这样做（注意，两个主题中的头文件只需调用header.php) -

get_main_header();
get_header();

我建议你读一下Function Reference for switch_to_blog() 了解更多信息。

更新

我忘了提到你还需要将你的标题样式与它自己的分开header.css 在主题A中归档，然后在主题A和主题B中将其排队。

我已经更新了我的代码示例来反映这一点。