Alter Color of Thickbox Effect?

When you click the toolbar button to add a link (or any number of other activities), it employs a Thickbox 'popout' effect where it dims (or, in this case, brightens) the screen in order to emphasize the activity you're doing.

Unfortunately, the white effect washes out the screen and looks horrible on my site. I'd like to make it more traditional so it dims the background rather than brightening everything. How could I go about doing this? I've searched exhaustively on the forums and in the docs and samples, but I simply cannot seem to find it. It's possibly I've simply looked over it somehow, but if anyone can point me in the right direction, I'd be very appreciative.

Thank you in advance!