A Zen Cart template is a series of files. The simplest ones are just variations on the CSS in Zen Cart, but they are variations which use the available classes and IDs in the Zen Cart code. A CSS template not designed for Zen Cart wouldn't use those class names and ID names, and therefore wouldn't work.

If you really like one of those templates, you could create a custom template to match it more or less easily, depending on the template you select.

You can read more about ZC's template process in the Tutorials/FAQ section, link in the top line above.

Rob