What is a stylesheet? (A Primer)

The Stylesheet is the standard way to define the look of your site, from fonts and colors to the complete layout of a page.



Style Rules

Stylesheets are made up of a list of style rules describing how documents are presented in a browser.

Selectors

Style rules are formed as follows: selector { property: value }

An element or type selector is the tag name of an HTML element and matches all elements of that type
h1 {
font-size: 1.5em;
}
This example sets the size of all h1 elements.

An id selector is preceded by a pound sign (#). An id selector can be used only one time in a document.
#headerWrapper {
margin: 0em;
padding: 0em;
}
The above example describes the "header" portion of your document.

A class selector is preceded by a period (.). A class selector can be used  more than once.
.important {
font-weight: bold;
}
Anything with class="important" will be bold.

Stylesheets in Zen Cart

In Zen Cart the list of style rules is saved in a file called stylesheet.css.
By default, stylesheet.css is stored in includes -> templates -> template_default -> css

Advantages of Stylesheets
HTML4 for Dummies has a helpful article explaining the concept of stylesheets and the advantages of using them.