Since the overlay style card renders "inside" your page, it inherits CSS from the page it's in. I found this declaration in your style.css file that makes the text white in all form fields:
textarea, input {
background:#333;
border-color:#555 #555 #555 #555;
border-style:solid;
border-width:1px;
color: #fff;
}
Adding a declaration to override that for input fields which are inside the cart's main DIV should do the trick:
EJEJC_window input {
color: #000;<br />
}
Handy tip: To figure this out, I used the View Source Chart extension for Firefox on your page while the cart was displayed, so I could examine the cart's HTML structure and tailor CSS accordingly.
