There is no way to control colors of DatePicker
Date picker colors can be changed by changing an app's theme today (and we just released some great new themes: https://powerapps.microsoft.com/en-us/blog/themes-update-adds-new-office-themes-and-contrast-complia...) -- that said, agreed that makers need more control over specific properties. Flagging @FilipK whose team has some work planned for date picker controls and upgrading the status here.
I think even if we didn't have full control over the color scheme, it would be good if we could apply themes to different elements without having to apply them to the whole app.
Maybe an advanced section of the theme picker with checkboxes for the elements it can be applied to?
Agree that we should be able to change the colors in the calendar to match the color scheme used elsewhere in the app. I use our corporate color pallette for all of the controls, etc. When the calendar pops up and uses a completely different color, it negatively affects the consistency of the color scheme.