Hi,
Thanks for your message. We are not sure to have understood exactly the button you are referring to. We assume you are referring to the front-end section of your website, and since you mentioned “under the availability calendar”, we believe you are referring to the “Room Details” page. However, it is not clear what “Details” button you mean.
Our suggestion is to define some preferred colours directly from the Configuration page of Vik Booking in the wp-admin section, so that all buttons displayed in the front-end will apply that colour, unless your Theme or some other plugins are forcing another style for it.
We invite you to get in touch with our team through our website should you keep having problems applying a specific colour to a button so that you can either apply a custom CSS rule through the apposite customization file in Vik Booking, or you can use the global colour settings. In any case, if you can contact us through our website you can also attach screen shots and links that will help us understand better the button you refer to. The link you posted is for a back-end page.
Thank you,
The VikWP Team