Unfortunately I have no Hilla example. Can you share a minimal example?
If you are styling a part, you don’t need to prefix the selector with :host (if you do you need to include a space after it). Just use [part~="..."]
The correct sub-components are
vaadin-date-time-picker-date-picker and vaadin-date-time-picker-time-picker
@resilient-meerkat @adaptable-uakari It worked with vaadin-date-time-picker-date-picker and vaadin-date-time-picker-time-picker. Thank you for the help guys 