Colors are controlled by the stylesheet.
In includes/templates/YOUR_TEMPLATE/css/stylesheet.css, look for
body {
margin: 0;
font-family: verdana, arial, helvetica, sans-serif;
font-size: 62.5%;
color: #000000;
background-color: #e5edf5;
}
and change it to
body {
margin: 0;
font-family: verdana, arial, helvetica, sans-serif;
font-size: 62.5%;
color: #ffffff;
background-color: #000000;
}
There might be other places that'll need to be changed, but that will give you the idea.
The second example is part of the stylesheet in use on the Fan Odyssey site in my sig.
The best way to do this is to unzip Zen Cart on your local drive (your computer). Edit php files and the stylesheets (css) is with a good text editor like Notepad++ or Crimson Editor. Use an FTP client like Filezilla to upload them to the custom folders on your site.
A handy tool is the Firefox browser with the web developer addon (or the firebug addon) which will allow you to tinker with the css and see the results on the fly. You'll have to save your changes locally and FTP them to your site.
(Filezilla, Notepad++ and Crimson Editor are all free downloads on the net)