How can I (for the long term) style the classic editor’s TEXTAREA?

I think I am following your question.

You can enqueue a stylesheet only for the admin area and then add your styles to that. The following code will enqueue a stylesheet in a “css” folder in the root of your theme.

function my_admin_styles(){
    wp_enqueue_style(
        'admin_css', 
        get_stylesheet_directory_uri() . '/css/admin-styles.css', array(), filemtime( get_stylesheet_directory() . '/css/admin-styles.css') 
    );
}

add_action('admin_enqueue_scripts', 'my_admin_styles');

All you need to do is create a css directory in your themes root, add a file called admin-styles.css and add your styles to that.