I'm trying to apply icon themes in my Ubuntu 18.04 setup, and for some reason, certain icons won't have icons applied to them, even though I know they're icons designed for said applications. When I was on Ubuntu 16.04, I used the numix circle icon theme, and there was a circle icon for Discord and Gimp. Now, with the same icon theme, those icons won't be applied. I tried the papirus icon theme as well, with the same result. How can I fix this? I applied the icon theme using the gnome-tweak tool.
If you installed the applications via snap, there is a problem with snaps accessing some global settings like themes. See https://forum.snapcraft.io/t/use-the-system-gtk-theme/496/3 for a more technical description.
You can change the app icon yourself though, and pick the Numix one. You have to start Files/Nautilus as Super User with
find the app, right click, properties, then click on the app icon, look for the Numix one and apply.