Hi,
It's in the file media/com_hikashop/css/notify-metro.css which contains the main CSS of the notification box system although it also generates some CSS dynamically.
But that doesn't matter.
If you want to add custom CSS to it, then you can add your custom CSS to frontend-custom.css and it will apply to it. Make sure you use !important in your CSS so that it gets the priority and that will do.