Customize style booking form

Hello, I would like to customize the appearance of the booking calendar. For example, I have customized the buttons globally on my website. However, they are still unformatted on the booking form. The first button after the period is selected is formatted. After that, the selection for the apartments opens, but unfortunately the formatting does not apply to it. Is there a way to include a CSS code within the booking form so that the buttons are identical everywhere?

Thanks in advance for your help.