Lightning Design System Components for Web's Datepicker can only display ranges, but not single selection.
Days can be overridden with Range Start / Range / Range End, but a symbol for a single selected day as displayed here is missing:
https://www.lightningdesignsystem.com/components/datetime-picker/#Date-selection
The solution is quite easy and quick: Just make a copy of Datepicker / z-nested / Today and change the background to blue and the text to white.