By default they should already be loading after the core admin styles. I suspect the CSS isn’t even being loaded because you have an invalid parameter in your wp_enqueue_style() call. Its parameters are the same as wp_enqueue_script(). Where you have 99 is where dependencies are actually defined. So WordPress is looking for 99 to be loaded, not finding it, and skipping the enqueue.
So I would re-write that as follows:
wp_enqueue_style( 'datatables-css', '//cdn.datatables.net/v/bs4/dt-1.10.16/datatables.min.css', 'datatables' );
What this does is loads the stylesheet but only if the JavaScript file is loaded.