The background image on the page is supplied with the theme and is the default image for new pages. To change the default picture replace the file "image.jpg" in the theme > images folder with your image which you've renamed to "image.jpg".
To change the background image on any one page simply choose to edit the page properties, choose Image Background and then using the file selector select an image from your files directory.
The unique navigation system is controlled by the autonav block. Upon first install you will need to activate the "Metropolis" auto nav custom template. Then alter the autonav options to show all pages on all levels.
The colors are modifiable through the customize menu in the dashboard of your site.
The navigation items width is controlled by the css. Every third item will display as double wide as shown. This behaviour is modifiable if you know css. Otherwise please contact me and I will modify it for you.