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;
}