Yesterday gtk-3.20.9 stabilized for amd64, my system updated from 3.18.20. This made nemo look weird and drop massive amount of such lines in console : (nemo:903): Gtk-WARNING **: Theme parsing error: gtk.css:1859:47: The :insensitive pseudo-class is deprecated. Use :disabled instead. emerging it again solved it. x11-themes/arc-theme should somehow update after gtk update. Thanks Arnaudv6
This is a known issue but I'm waiting on upstream to act. Please voice your interest there. https://github.com/horst3180/arc-theme/pull/436