Presumably you're referring to the line of CSS code suggested on this help page:
http://www.e-junkie.com/ej/trouble.buttons.cart-css.htm
That was really intended as just an example suggestion to get you started on the right track; it's not really possible to offer a more specific solution without seeing a page where the problem is happening, but you may need to experiment with adding CSS selectors for elements nested inside that main cart DIV, e.g.:
div#EJEJC_window,
div#EJEJC_window div,
div#EJEJC_window table,
div#EJEJC_window td
{background-color:#FFF;}
Accordingly, I've updated our help page with this expanded example.