Hello Paul J,
There are no separate skins for RadDatePickers. These controls use the RadCalendar and RadInput skins. So basically you need to create one custom RadInput skin for the datepicker textbox and one custom RadCalendar skin for the datepicker Calendar (and TimeView) popup. (Both custom skin should have the same name).
There are a few "datepicker-only" styles, which are located at the end of the Calendar CSS file. Most of them should be the same for any skin. The only thing that may need modifying is the Calendar sprite image, which contains the images used for datepopup and timepopup buttons.
If you inspect any of the non-embedded Calendar CSS files, which are located in the "Skins" subfolder inside your RadControls installation location, you will see what I mean. Let us know if you need more information.
the Telerik team