Hi im new to zencart but hopefully understand what you are doing wrong. What you will need to do is create a custom template and work of that rather then the default one. the reason to this as i understand it is if you require a upgrade to zencart then all of your customisation will be lost.
What you need to do is this:
go to SHOP/includes/templates(SHOP being the name of the directory you uploaded zencart to)
Create a new folder and call it what you like(mytemplate)
Now go to default template and copy the css into the new folder you created
........................Ive just found the link i was looking for................

Follow these steps and work under your CUSTOM template
cheers