You're in luck because I just recently did this to a RadCalendar instance in my latest app. You can override the control's system resource bindings by explicitly setting the BackGround, Foreground and GridLine colors.
Keep in mind that if you do this, you lose RadCalendar's ability to adapt to the user's preferences. Take a look at my attached screenshot. The phone is set to light theme and the RadCalendar defers to the explicitly set color values.
Here is my code for the instance you see in the screenshot