Hi again,
After playing some more with modal window, I agree that the background is working fine. There is a little thing though (very minor). If you open a modal window when the page (the page calling the modal window) is bigger then the browser and a vertical scrollbar is visible, the modal window will leave the vertical scrollbar and display the modal background in the remaining space (leave the scrollbar intact). This is peerfect behavior. It works perfectly in Chrome, Firefox and IE 8. However, in IE9 and IE10, the the modal background does not seem to take the width of the scrollbar into consideration and a horizontal scrollbar appear at the bottom.