Inevetably, once you have changed the ControlTemplate of the RadDateTimePicker, you will need to maintain that customization and update it if our release contains a breaking change. The good news is - such breaking changes are very rarely introduced.
The problem you are having with the theme is that Expression Blend generates always the default theme for RadControls so the Office Blue one was ignored in your case. Basically you have made your customization upon Office Black and have applied Office Black, instead.
You could get the Office Blue theme resources from your installation folder for RadControls and Themes\OfficeBlue\Themes\Office\Blue - DateTimePicker.xaml. For your convenience, I have attached it here. Please, make your customization in that resource dictionary.
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