Right-click on elements, then "inspect" them in your browser console where you can view HTML and the related CSS... as well, and that's the important part, the CSS files where it's coming from.
All modern browsers have this console built in, and e.g. Firefox could be extended with the glorious Firebug. See also documentation here, where it's referred to:
www.hikashop.com/support/documentation/1...the-display.html#css
All of this is basic, essential web design stuff, not rocket science 
In your site, there is no HikaShop CSS file in use. It's not even loaded in the <head>. Proof: check in the HikaShop backend -> System -> Configuration -> tab Display -> section CSS. What's the setting for the option "Front-end CSS File"? I bet it's "None".
That's generally ok in your case, because all CSS, means all styling of your shop, is coming from your template's files. Means that you should use your template's tools to change it to your needs. And - worst case - go back to your template provider if something is "wrong".
BTW, as with style, it's similar with layouts on your site: there are customized shop layouts that came with your template. Just FYI, you'll find those by following
www.hikashop.com/support/documentation/1...-display.html#layout
: in the list of views you'll find those which a a little trash bin on the right. Those are the views where overrides are in use - overrides made either by your template provider or by yourself.
Careful: Don't click the trash bins! It would delete those view overrides! So, nothing there without first backing them up!