First, don't think of Zencart the same way you think of flat HTML sites. There are very few static pages in Zencart. Most all pages are built on-the-fly.

Zencart's CSS is in one or more stylesheet files in includes/templates/YOUR_TEMPLATE/css.

If you post a link to the site we can advise further.

Best advice:

Install the Firefox browser and the Web Developer plugin for Firefox. You can use those to see where in the stylesheet you change the styling of the various page elements.

Ctrl-Shift-Y will allow you to hover over a section of the page and see the class and id names for that section in the box at the top. The statements in the stylesheet that begin with # are id's and the ones that begin with dots are classes.

Ctrl-Shift-E will open the CSS editor and allow you to edit the stylesheet and see the changes in real time without changing anything permanently. An essential tool.

Some prefer Firebug, which does much the same thing.