Hi Friend,

Use the same css classes which are overridden as

<style>
        .modalBackground {
                background-color:Gray;
                filter:alpha(opacity=70);
                opacity:0.7;
        }


        .modalPopup {
                background-color:#FFD9D5;
                border-width:3px;
                border-style:solid;
                border-color:Gray;
                padding:3px;
                width:250px;
        }
    </style>

As given in http://forums.asp.net/t/1188275.aspx/1
Also use check other classes using IEweb developer tool and check the css class names and use that.
Use the css http://www.asp.net/ajaxLibrary/AjaxControlToolkitSampleSite/StyleSheet.css to copy existing classes.