Appointment form is very simple and modern looking out-of-the-box and works perfectly fine on any WordPress theme. However, there is always a need to change the color scheme on the front-end to suit your existing theme design.
Customizer
You can alter main color scheme for your appointment from directly from the Appearance > Customize panel in admin. Color scheme options are available from the WooCommerce > Appointments panel in the customizer:
Additional CSS
If you are not using a child theme for WordPress or your theme does not allow you to enter custom CSS, perhaps you can use the Additional CSS field inside WordPress Customizer. The best way to edit the styling is using browser developer tools, i.e. Chrome style inspector.
Here are the CSS styles for the different appointment form elements:
Copy and edit the elements you want to change.