There is a problem with the Crux theme. The dropdown arrows beside the forward and back buttons in Nautilus seem to be missing their theme. I will attach a small image to this bug to show you. The rollover image on the arrows works fine, so it is just the default state that is not themed properly.
Created attachment 1854 [details] Image to show Crux problem. Here is an image showing the problem with the Crux theme.
Yep, it is something releted with crux itself, an upstream issue...