Hello Roman Wack,
Watch carefully the video. For example Panels are used to layout. The MonthViewPanelTemplate is the template for the Panel that will be used to layout the items of the RadCalendar in month view. This is like editing the ItemsPanel property of an ItemsControl. I suppose what you want to edit is the MonthViewStyles of RadCalendar instead which is like the ItemContainerStyle of a ListBox.
After you create style for RadCalendar you can just apply it on the CalendarStyle of RadDateTimePicker.
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