I'm using Unity 7 on Ubuntu 18.04 LTS. I've installed and set the Numix Square icon theme and restarted Unity. Nautilus seems to still show the default empty file icon for all files and folders. It does show image thumbnails, however. Changed icons with Unity Tweak Tool.
I did not have any icon themes installed with folder and file icons so it fell back to the default empty file icon from the GNOME system theme.