Hi Phenicy Informatica Srl .,
This is very strange issue and we aren't able to reproduce it. However you can use the sample code below to control the properties of the RadCalendar used in RadDateTimePicker. Here is the code snippet:
|RadCalendar calendar = (this.radDateTimePicker1.DateTimePickerElement.GetCurrentBehavior() as RadDateTimePickerCalendar).Calendar;
|calendar.ThemeName = "Telerik";
I hope this helps you. Please contact me if you have any other questions regarding RadDateTimePicker.
the Telerik team