This is strange but when I first load the calendar the tooltip font is too dark to read it when hovered over. I saw in another thread to add .tribe-events-tooltip .tribe-events-event-title {color: #FFFFFF !important;} to the CSS which I did and no change.
Here is link to test calendar http://www.genxmd.com/events/
Strange thing is if I click on the next month then over it looks fine. If I then go back to the previous month like it was when I first loaded page it is fine. Only when they page is first loaded does this happen. Any thoughts?