[GNC] Can't find GNC_CONFIG_HOME or USER_DATA_HOME on Linux

[Sam Quintanar]

I read the documentation that suggests I use a gtk-3.0.css to change the colors of the Register.
This file should be in the folder GNC_CONFIG_HOME to keep color changes limited to GNC, but such a folder does not exist on Ubuntu 18.04.
Am I expected to create the GNC_CONFIG_HOME folder, and then create the gtk-3.0.css inside it?