Lightbox position - html

Question

I have a Light Box Code that works great! You click on a button and then there is an overlay of a black screen and a content box. I am wanting to know if there is a way to make the light box or white_content in the CSS below appear on the screen based on how far down the user has scrolled down the page.
Basically, I am wanting to have the white_content appear in the middle of the view able screen.
http://s1309.beta.photobucket.com/user/mievan123/library/Light%20Box
The first image is showing the Light Box centered on the page and this is what I want. I am scrolled all the way down the the bottom of the page.
Also you can see it in action at http://www.green-panda.com/solutions.html
The second image is showing the Light Box barely visible on the page. I scrolled up just a little as you can see. The Red Outline is where I want the Light Box to move to when i'm at that position.
Would a screen recording help make this question more clear?
All of my code is provided below:
My CSS for the light box
.black_overlay{
display: none;
position: fixed;
top: 0%;
left: 0%;
width: 100%;
height: 100%;
background-color: black;
z-index:1001;
-moz-opacity: 0.8;
opacity:.80;
filter: alpha(opacity=80);
}
.white_content {
display: none;
position: absolute;
top: 50%;
left: 25%;
width: 50%;
height: 50%;
padding: 16px;
border: 16px solid green;
background-color: white;
z-index:1002;
overflow: auto;
}
The script for opening the lightbox
<p align="right">
<a href="javascript:void(0)"
class="button"
onclick="document.getElementById('light').style.display='block';
document.getElementById('fade').style.display='block'"
>Contact Us</a>
</p>
The actual lightbox coding
<div id="light" class="white_content">
This is the lightbox content.
<a href="javascript:void(0)"
onclick = "document.getElementById('light').style.display='none';
document.getElementById('fade').style.display='none'"
>Close</a>
</div>
<div id="fade" class="black_overlay"></div>
JSFiddle

Answer 1

You can't center it with the width and height being percentages (at least not without using JS).
You can however set a static height and width and center it like this:
Use top: 50%; left: 50%; and static, negative top and left margins, which should be half of the width/height of your element.
.white_content {
display: none;
position: fixed;
top: 50%;
left: 50%;
margin: -182px 0 0 -182px;
width: 300px;
height: 300px;
padding: 16px;
border: 16px solid green;
background-color: white;
z-index: 1002;
overflow: auto;
}
JSFiddle

Answer 2

I have created a demo for you here. I am sure that will help you to achieve that you want.
Following is the code that I am using in the demo:
/* This parent can be any width and height */
.block {
text-align: center;
}
/* The ghost, nudged to maintain perfect centering */
.block:before {
content: '';
display: inline-block;
height: 100%;
vertical-align: middle;
margin-right: -0.25em; /* Adjusts for spacing */
}
/* The element to be centered, can
also be of any width and height */
.centered {
display: inline-block;
vertical-align: middle;
width: 300px;
}