Could you please share which theme you are using by default. I suppose you are using Windows7 theme. The button you are refering to is black by design, but its color can be changed by editing the template. I have attached a sample project where I have copied the Windows7 theme style to the App.xaml and have applied the CalendarStyle
to the RadDateTimePicker. I have changed the CalendarHeaderButton
foreground to Red in the template(you can find the change by searching "NOTE" comment). You can change the color to whatever you feel appropriate.
the Telerik team
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