How to properly add my styles overriding current styles

You will need to build a child theme to prevent your changes being overwritten when the theme is updated. This child theme needs only to consist of a functions.php file and a stylesheet that you don’t name style.css to prevent it from being loaded automatically.

In your functions file you load the stylesheet only for this specific page, like this:

add_action ('wp_enqueue_scripts','wpse278950_add_my_style');
function wpse278950_add_styles () {
  if is_single ('slug-of-your-page')
    wp_register_style ('my-style', get_stylesheet_directory_uri() . '/my-style.css');
  }
}

Note: I didn’t test the code, it might need debugging.