Changing a css file

[watcherob]

Due to circumstances I have to take over maintenance of a website using Zen Cart. I'm sorry if this question has been asked millions of times but i'm in a bit of a hurry so I really hope someone can help me.

The website starts of being a standard html site with css. As soon as you enter the shopping part of the site (the Zen Cart part) the css changes. I have looked for hours but i cannot find out where I can change the css file within the Zen Cart system or maybe within a loose file somewhere.

If I look at the source code in the browser it looks like a standard index.html file but that I cannot find that file, probably because it's built through Zen Cart? I know that Wordpress does the same thing.

You would make my day if you could help me out with this. Thanks!