How can I move my custom css to the very bottom of the head section?

Set the priority argument on add_action(), the 3rd argument, to something high:

add_action('wp_enqueue_scripts', 'add_custom_css', 20 );

The default is 10, so most styles will be loaded at that priority in the order that the plugins are loaded, so 20 will be after most of them. Increase or decrease that number as required.