The docs note that the WebPopupMenu will not respect a style in Safari because of some stuff Safari does. The work around suggested is to make the background not white (i.e. 255,255,254). The problem with this is that anything else using the style (e.g. a label) now also has an almost white background.
What I found is that you can probably set the background to anything and set the Opacity to zero (0). This appears to work.
-Bob