There are many classes available in the code in Zen Cart (the code created by the interaction of the many php files, which is then rendered by your web browser). If the class you want to style is not already in the stylesheet, just add it and style it.
You can easily see the class you want to style by using a developer's tool in the browser -- Firebug in Firefox is often recommended. Or you can even find it by closely examining the code using the View Source menu-item in your browser.
In your case, since the main background is black, I'd recommend styling the centerColumn class to start with. It's probably already in your stylesheet, but if not, just add it. I always add a comment at the bottom of the default stylesheet then add any additional classes below that. Makes them easy to find later, and ensures they are the last read styles just in case tthat class is already styled up above in the stylesheet.
By the way, you need to seriously consider doing some image optimization. The images of people on your home page in particular are HUGE and are causing the site to load slowly.
Rob
Breakfast: the most important donut of the day.