I was trying to edit an SVG with Inkscape, and I found this:
Normally look like this:
The rest of the icons but
- signs seems to work fine, it only affects GTK applications.
To solve this, I added the following packages for my NixOS configuration
pkgs.gnome.adwaita-icon-theme pkgs.gnome-icon-theme pkgs.hicolor-icon-theme
Now you will have GTK icons rendered correctly.