Why do you need to be able to control the look of the date picker (or any other input element)?

If every website used the same, it would be much more convenient than the current situation, where each date picker uses a subtly different mechanism to select the individual fields and then confirm the selected date. Most do not even support keyboard input.

Okay, please write a proposal for how to select parts of this widget with CSS. Remember, it must be compatible with all major browsers and across all formfactors (mobile, desktop, 10-inch, automotive, etc.). I'll schedule an appointment for a review in 2020, since I don't expect you to be finished earlier than that.