Thank you for your question.
RadDateTimePicker is a compound control and it uses the theming of RadDateTimePicker (editor part) and RadCalendar. Therefore, in order to make a theme for RadDateTimePicker, you should create two themes, one for RadDateTimePicker and one for RadCalendar (as you have already did). An essential step in this scenario is that both themes should have one and the same ThemeName property. Then, you should just load the theme files
in your project and your theme should be applied correctly.
If you need additional assistance, feel free to contact me.
the Telerik team