I've tried that in the master page and also within the content section of the sub page, but it doesn't seem to work.
filter: progid:DXImageTransform.Microsoft.Alpha(opacity=50) !important;
opacity: .5 !important; -moz-opacity: .5 !important;
If complains about having text between the opening/closing tags in the head section. I've also tried adding it to my modified skin (called ptcradwindow), but it also didn't seem to make any difference. I set the values to 10, .1, .1 respectively and 90, .9, .9, but none of the values made any difference.