With some elements modified with userChrome.css, some elements can use Windows Aero styling by using -moz-win-borderless-glass. On Mozilla's new styling for popups (Downloads and tab preview popup for example,) when -moz-win-bordless-glass is applied, a Windows Aero border is applied (albeit very small,) but the background color of the popup is black and does not seem to be modifiable with the browser toolbox and the element cannot be made transparent to take advantage of Windows Aero styling.
Given that the border that is applied uses Windows Aero styling, these popups should be able to have a transparent background rather than black to take full advantage of Windows Aero. Is there a way to fix this? Pictures are attached, You'll need to zoom in and look very closely for the Aero border.

