Customise colours in Bookings

Hi all,

You can see here that the difference between a date that can be booked and a date that can't is a slightly different grey colour. This is pretty terrible usability:

Ideally I would be able to change the background colour for any date that has the class .bookable:

Can anyone tell me if it is possible to customise Bookings in this way? Is there anything else I can do so it's evident to people which dates can be booked?

Cheers!