CSS for email capture pop-up background

Hey everyone,
I’ve been fiddling around with this for a couple hours and can’t seem to figure it out so I thought I’d see if one of you CSS whizzes has an answer! I’m working on the website https://www.drincredible.org and want to add a background image to the email sign-up box that pops up on the homepage. I’ve tried simply adding HTML to the campaign editor, but to no avail, and am now trying to use Firebug to isolate what I need to change in my custom CSS editor. So far I have tried:

div.icegram .ig_clear_fix {
	background-image: url("https://www.drincredible.org/wp-content/uploads/2015/05/IMG_5154.jpg");
}

This didn’t do anything

.ig_popup .ig_container .ig_bg_overlay {
	background-image: url("https://www.drincredible.org/wp-content/uploads/2015/05/IMG_5154.jpg");
}

This also did nothing

.ig_popup.ig_inspire .ig_container {
	color: none;
	background-image: url("https://www.drincredible.org/wp-content/uploads/2015/05/IMG_5154.jpg");

This managed to make MOST of the background the image that I wanted, but left a thick border around the sign-up box in the original background colour.

I was then able to isolate that box (I think) on Firebug:

.ig_form_container::before {
    content: "";
    margin: 0;
    padding: 0;
    position: absolute;
}
.ig_form_bottom .ig_form_container.layout_bottom {
    display: block;
    height: auto;
    width: 100%;
}
.ig_popup .ig_form_container {
    float: left;
    padding-top: 2em;
    width: 30%;
}
.ig_form_container {
    box-sizing: border-box;
    display: none;
    height: 100%;
    line-height: 1.1;
    padding: 1em;
    position: relative;
    text-align: left;
    z-index: 10;
}

But am now at a loss of what to change. I changed the box background to my image, but that of course resulted in that specific area having its own (identical) background. I think there much be some global way to fix this but I don’t know how!

Anyone have any suggestions?
Signed: eager but clueless CSS newbie