Thank you for the feedback. Indeed, such behavior exists. It has been logged and will be further investigated. We are sorry if currently the issue causes any inconvenience.
If you do not need to view the template structure of the RadDatePicker control, you can simply Rebuild the project and continue a normal customization process as you will have access to needed resources. If this is not the case, I would suggest that you make your customizations in place (i.e. in the same UserControl) and, when done, move them in a separate resource dictionary.
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