Ugly dark icons in gthumb and other GTK3 apps

[robinc]

Hi
I have been struggling with what I guess is a .css thing with a couple of applications.
gthumb is one I use the most and it is the horrible dark icons that seem to be a symbolic link to nowhere as I cannot find them on my system - new installation of 3.6
That along with the border that does not honour the window manager selection is just annoying - OK it works but I'd like to get it all looking as though it belonged together.
I have researched .css but have to admit it is way above my understanding. I see what it is doing but lack the skills to make a working solution. It is possible that it needs more Gnome components to be installed - rather not though.

[trinidad]

You have to choose a desktop default icon set that is available to gthumb. Faenza and/or adwaita should work.

*Actually after opening your file with gthumb I just checked this in Debian 9 gnome default and the icons you have are the default for the show folders function. They are light gray on the dark theme and dark gray on the default light theme. The issue is NOT because gthumb is a gnome app and you are running it in XFCE. It just is how it is. Probably for graphic memory preservation and/or usable camera icon signing.

TC