[Theme: Manifest] How can I hide popup overflow?

Hi,

I want to hide a popup overflow and I’m failing in doing so.
It suppose to be simple but I can’t modify it properly.
1. You can see the problem in the following picture:

2. If I change the overflow in the firebug from ‘auto’ to ‘hidden’ then
I get the result I want. The problem is that I don’t know how to modify the css file on my server. I’ve already defined the overflow to be hidden but with no luck.

.nyroModalCont {
	position: absolute;
	overflow: hidden;
	border: 4px solid #333;
	margin: 25px;
	background: #fff;
}

Can anybody help me with that issue?
Thanks and regards,

Kfir Silis