Hi! It looks like the datepicker in Niki’s file comes from setting a text field to the “Date” type; this allows the browser to display the datepicker that it would typically show for date fields. The datepicker on “Date” type text fields isn’t customizable in Axure RP, but if you’re interested in customizing the appearance of your datepicker then I would recommend building a custom one:
https://www.axure.com/support/training/custom-datepicker-tutorial
Hopefully that helps!