Correct way to apply css styling?

The default styling comes from media/com_hikashop/frontend_default.css. I think I'm right in saying this is overwritten when you upgrade/update hikashop? Is there a way to change hikashop to use a style sheet which isn't overwritten when Hikashop is upgraded/updated?

Yes you're right and yes there is a way
You can edit the CSS via the interface in the Display tab of the configuration of HikaShop. That will create a custom CSS file in the same folder which will be used instead of the default one and that file won't be overwritten when you update/upgrade

Aha! Perfect, thanks.