You will have to edit both the DateTimePicker.xaml and the ButtonChrome to change the color. Note that the calendar button use ButtonChromes within to render the mouse over and selected states so you will have to customize the button chrome and set the custom style as style for the ButtonChrome in the calendar buttons.
Also you will have to take care for the composite theme keys that are set on the style and use your own or make them implicit by removing the 'x:Key' attributes from the Styles.
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