I tried out and installed XnviewMP under a KDE-Distro (KDE Neon).
The folder-icons that are used here seem to be "oxygen", so the (older) default-icon-theme in KDE.
( I attached another screenshot here).
So the two default icon sets are choosen:
Gnome: Adwaita
KDE: Oxygen
How can we change this ?

- Bildschirmfoto_2017-08-28_08-18-56.png (24.33 KiB) Viewed 901 times