Hi, Michael!
The Hotel Booking plugin usually inherits styles from the active theme, including styles for availability calendar. If you would like to change its style, you will need to do that by overwriting default styles via custom CSS. Depending on the elements you would like to change the styles of, you will need to overwrite specific classes. You may find some examples in this topic. To get further assistance with specific calendar styles, I would recommend submitting a request to our Help Desk.
Regards,
Andre