This happened right after upgrading to Ubuntu 18.04 on my father's PC from Ubuntu 16.04.
Changing the icon pack or the GTK theme does not work. The only way I get to see the icons again is by disabling auto-coloring based on wallpaper using the Unity Tweak Tool.
Does anyone know why and how it happened? It'd be great if the icons are visible again with the auto-coloring enabled.