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.