User blog comment:Craiglpalmer/Introducing Fandom/@comment-4347751-20160125181327/@comment-1251315-20160125183941

.inverse refers to when the bar is in the black state after scrolling down. That class is not present when the bar is white, so it's likely for any custom CSS to not register that it's there without the .inverse class being selected separately. I didn't include it in a couple of classes myself and as a result, the inverse bar displayed differently to the non-inverse bar.

Of course, it's quite possible that some changes got made to the classes so now that .inverse isn't really necessary to be declared.