The Banner and Colour section allows you to manage those exact things. By default there is a BIMobject banner already added to any newly created product or project site.
The image above shows what parts of the product site the colours will affect. Also if you wish to use your own banner, just drag and drop the banner image into the “No file chosen” box.
Banner images cannot exceed the 150 x 800 pixel dimension and should be in a png format that allows for transparency for the best user experience.
Don’t forget to save your changes when you're done.
Below is an example of changing the colours:
and the resulting product site