In addition, certain files will be read when GTK+ is initialized. First, the file `$XDG_CONFIG_HOME/gtk-3.0/gtk.css` is loaded if it
exists. Then, GTK+ loads the first existing file among `XDG_DATA_HOME/themes/THEME/gtk-VERSION/gtk.css`,
`$HOME/.themes/THEME/gtk-VERSION/gtk.css`, `$XDG_DATA_DIRS/themes/THEME/gtk-VERSION/gtk.css` and
`DATADIR/share/themes/THEME/gtk-VERSION/gtk.css`, where `THEME` is the name of the current theme (see the
gtk_theme_name setting), `DATADIR` is the prefix configured when GTK+ was
compiled (unless overridden by the `GTK_DATA_PREFIX` environment variable), and `VERSION` is the GTK+ version number. If no file is found
for the current version, GTK+ tries older versions all the way back to 3.0.
In the same way, GTK+ tries to load a gtk-keys.css file for the current key theme, as defined by
gtk_key_theme_name.