As the name states, "in-line" styles are style attributes set in the HTML or PHP code. So, to get rid of them in HikaShop, you'll need to edit views - and probably many of them, depending on which ones you are using. However, HikaShop offers you a way in the backend to edit these views without messing with core files, so you'd be still ok when updating one day.
As for all other styles, they are coming from a CSS file, as very common these days. You can override that, and the HikaShop backend offers an easy way to create a custom override CSS file - again, safely regarding updates.. The most simple, fastest but also "brutal" way to get rid of all that would be to completely delete all content of that custom file and save it empty. The result will then entirely depend on your template, and you might be up for a (bad) surprise. Well, you can always delete the custom file and be back to default, then rather work on one style at a time...
Other than that, it's always worth reading documentation, wouldn't you agree? There's a whole lot on this site - click on "Support" here above again and browse from there.