When DateSelectionMode is Month, RadDateTimePicker has been opened and theme is changed, there is an InvalidOperationException. Workaround: 1. Set the DateSelection mode to Day - <pickerInstance>.DateSelectionMode = DateSelectionMode.Day; 2. Clearthe merged dictionaries for the theme 3. Add the new dictionaries 4. Set the DateSelection mode to desired - <pickerInstance>.DateSelectionMode = DateSelectionMode.Month; Available in LIB version: 2017.2.710