Please, excuse me for not being clear enough in my previous reply. This is not an actual class name, you should replace it with the existing class of a DatePicker's wrapper from your actual page. If you want you can send me a runnable page reproducing the problem, so I can examine the exact layout that you use and advise you on the element you have to style.
On a side note For example, if you have a div wrapping the DatePicker you can try styling it like suggested:
If this does not help, though, please, send us your setup, so we can examine the layout leading to the problem further and advise you on how to fix it.