GTK+

The chosen theme controls controls how a GTK+ application looks.

Apparently, the themes can be configured in ~/.config/gtk-3.0/settings.ini.

~/.config/gtk-3.0 ~/.config/gtk-3.0/bookmarks ~/.config/gtk-3.0/settings.ini

/etc/gtk-2.0/gtkrc

~/.config/gtk-3.0/bookmarks stores bookmarks (for example used in Nautilus)