You can adjust the design of the booking form by going to "Settings > Booking form". Under "Manage booking forms", click "Edit design" for the booking form that you want to modify.
Here, you can edit the CSS, which means you can customize all aspects of the design, as well as remove elements from the booking form, such as the copyright text. You will need some basic knowledge of CSS to use this option.
Here is a few examples of Sirvoy booking engines customized by the hotel with CSS and HTML: