Listgrid checkbox Filter

I am using SmartGwt4.1 power version. I am unable to align the ListGridField (Checkbox) field in a filter row option for the last column in a ListGrid Component to be center, but by default it is aligning left. Please let me know how to set it.

ListGid field Code :-

ListGridField field1= new ListGridField("user");
field1.setTitle("<B>User</B>");
field1.setType(ListGridFieldType.BOOLEAN);
field1.setAlign(Alignment.CENTER);
field1.setCanEdit(true);
field1.setCanFilter(true);
field1.setWidth("*");