Gtk3 is there a way to custom the style of specific application.

[Simon Bernard <sbernard sierrawireless com>]

Hi,

  I would like to know, if it exists a way to change the style just for
one specific application with GTK3.

  In GTK2, I just set the environment variable
GTK2_RC_FILES=mycustomgtkrcfile/gtkrc.

  I search for a while, and I didn't found anything.
  
  The documentation[1] and this post[2] seems to present a way to custom
a theme for a user but not for an application. (I suppose setting the
value of $XDG_CONFIG_HOME, is not a good idea because it should be used
for other purpose than just styling?)

[1]https://developer.gnome.org/gtk3/stable/GtkCssProvider.html#gtkcssprovider-files
[2]https://mail.gnome.org/archives/gtk-list/2011-March/msg00108.html