Is there a way to add CSS overflow attributes to DIVS in Zen Cart?

My design does not fit the billing info/check out page. I would like to add a scroll bar to it so it fits the layout.

I tried adding overflow: scroll; and overflow: auto; to the #contentMainWrapper but it didn't do anything.

Here's a link to the page I am hoping to add the overflow style to...

http://openheartthreads.com/products...ain_page=login

Thanks in advance!
Robert