Go to Property Settings / Booking Engine / Colors and edit the colors on the list.
You can select the Booking Engine Theme (Classic, Dark or Light):
Or you can adjust each color manually selecting from the palette, for example, Header Color, Background Color etc.
It is possible also to use hex colors. Just search HEX COLOR on Google and select the color from the palette. A HEX color has a # and 6 signs, for example: #d3e4ed, #fcfcfc, #f6d9d9.
Copy the code of selected color from Google and then paste it into a selected area.
Each time you click Save and Preview you will see the new window with the changes.