Snippet to override the calendar CSS of woocommerce-booking-and-appointments plugin https://www.pluginhive.com/product/woocommerce-booking-and-appointments/

style.css

//Change the color of text  "pick Please a booking period" here.
.callender-msg{
	color:white;
}

//Change the color of the calendar here.
.ph-calendar-month{
	background:red;
}

//Change the color of labels for participant and resource here.
.label-person{
	color:white;
}

//Change width of the calendar here. 
.booking-wraper{
	width:90%;
}