Thanks, I misunderstood you.
The default (white background seems to come from
background-color: %color:On Secondary Container%
In the Rally theme I am using, the background of the DatePicker seems to be %color:Surface Variant%
, so adding this fixed my problem:
.daterangepicker select.hourselect,
.daterangepicker select.minuteselect
{
background-color: %color:Surface Variant%
}