There are no CSS themes for HikaShop for now. That's on the todo list and we will try to do that before the end of the year.
However, by default, the front end of HikaShop should adapt to the styles of your template. For example, if your template defines some styles for button, the buttons in HikaShop will use that style automatically.
For the module, you can also find a lot of options in the cart module's hikashop options in order to display your cart differently (not styling, but what information is or isn't displayed)...