The way I’ve done is this: copying to my CSS style sheet (through my Theme option “Edit CSS”) the following part, from the file ./availability/style.css.
Then I’ve changed background-color, color and text-decoration.
.wp-availability-month table tbody td.wp-availability-booked {
background-color: green;
color: white;
font-weight: bold;
text-decoration: none;
}
I dont′t know if it’s the most optimum way, but it works ?? Remember to erase the quotes if you copy-paste.