You can use the styling properties of it to change the color.

See this: https://docs.mendix.com/refguide7/date-picker/#8-common-properties
 

If this is not enough, you can inspect the DatePicker in the browser and see the classnames added to the DOM elements and then apply styles to them directly in the scss files that let you theme your modules. You can use this link to learn how to do it: https://docs.mendix.com/howto/front-end/customize-styling-new/