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
*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
All opinions expressed and all advice given by Trinidad Cruz on this forum are his responsibility alone and do not necessarily reflect the views or methods of the developers of Linux Lite. He is a citizen of the United States where it is acceptable to occasionally be uninformed and inept as long as you pay your taxes.