DatePicker arrows clickable but not visible

I'm using a datepicker for a field on my form, but the arrows are not 开发者_如何学Pythonvisible. If I go to the location where they supposed to be, I can click them to change the month, but the arrows themselves are not visible. What could cause this kind of problem?

The problem could be CSS cursor style set to none, something like this:

.calendarBox
{
   cursor: None;
}