The easiest way is to have a superimposed div with a a high z-index, with transparent background acting as an overlay. You could then have another div which is centered above the overlay(with higher z-index) and containing the list markup

CSS

#shim {
opacity: .75;
filter: alpha(opacity=75);
-ms-filter: "alpha(opacity=75)";
-khtml-opacity: .75;
-moz-opacity: .75;
background: #B8B8B8;
position: absolute;
left: 0px;
top: 0px;
height: 100%;
width: 100%;
z-index:990
}

#msgbx {
position: absolute;
left: 50%;
top: 50%;
height: 150px;
width: 350px;
margin-top: -75px;
margin-left: -175px;
background: #fff;
border: 1px solid #ccc;
box-shadow: 3px 3px 7px #777;
-webkit-box-shadow: 3px 3px 7px #777;
-moz-border-radius: 22px;
-webkit-border-radius: 22px;
z-index:999
}

HTML

<div id="shim"></div>
<div id="msgbx">inject list markup here</div>

To show popup

document.getElementById('shim').style.display=document.getElementById('msgbx').style.display ="block";

To Hide

document.getElementById('shim').style.display=document.getElementById('msgbx').style.display ="none";

I put together an overly simplistic jsFiddle demo of how a popup is made. Here is the magic:

//Display Popup method - you can modify this to add parameters 
function displayPopup(event) {
        //dynamically create a div, button, and inner text
        var p = document.createElement('div'),
            b = document.createElement('button'),
            t = document.createTextNode("Text and stuff");

    //add popup class to newly created div
    p.className += 'popup';
    //Add text to close button element
    b.innerHTML = "close";
    //Append text to popup div
    p.appendChild(t);
    //Bind click event to button element
    b.onclick = closePopup;
    //Append button to popup div, and append popup to document body
    p.appendChild(b);
    document.body.appendChild(p);

    p.style.display="block";

    //This function hides the popup
    function closePopup(event){
        p.style.display="none";
    }
}

It should by no means be considered complete or production material, but I hope it helps you get started on the right path to extending it to achieve the functionality you're looking for.