I checked the given URLs, and I see no One Page Navigation. There is the home page, Contact page, and the Team page.
Would you please kindly give us more detailed information about the problem that you are experiencing? By background, you mean the red color of the menu? Would you please give us screenshots of the behavior that you see which you think is not correct?
Meanwhile, I think it does not matter if you are using a multisite or not, as it is a matter of front end. The only thing that I can suggest at this stage is, to make sure you do not have duplicate IDs for your one page. And if you use the UberMenu, you should not use our One Page functionality mentioned here.