The standard ZC template system comprises:

a header
a left column
a centre column
a right column
a footer

The bulk of the layout and aesthetics are governed by the chosen template's CSS file(s).

So, in the CLASSIC template (the one ZC defaults to when you do a fresh, un-edited install), the template is drawn from the ORIGINAL files in:

templates/template_default/templates/

... and the stylesheets are drawn from

templates/classic/css/

Now, in order to take advantage of ZC's "over-ride" system, you ought to create YOUR OWN template system. (The over-ride system facilitates the process of custom edits to core files, protecting these "edits" from being over-written when you upgrade the software at a future date.)

There are tutorials and instructions for creating your CUSTOM template in the FAQ and WIKI sections of the forum. You could also upload a new template from the Free Add-ons section, or buy one from a 3rd-party vendor.

Once you have your CUSTOM template system in place, you will be able to safely make edits to core files.

Your existing XHTML page and its accompanying css will not, I'm afraid, help you very much, because ZC's template system and its CSS is likely to be very different from your current code.

ZC does not work with HTML, but with PHP, where the final HTML is "cobbled together" from scores of separate resources (including the database).

I have heard that some people manage to use dreamweaver to assist them when they "edit" their ZC template, but they are still working within the strict constraints of the ZC template code structures.