is it the whole pop-up that has transparent background, or just the date header, or just the list of events?
cal_popup_cg controls the whole pop-up, cal_popup_caption controls the date header and cal_popup_fg controls the list of events.
you need to add say, background-color: #ffffff;
i.e. instead of background-color: transparent; or maybe there is no rule there at all.
cheers