When debugging a program or testing a change to an installed version, it
is often useful to be able to replace resources in the program or a
library, without recompiling.

To support this, for debugging and hacking purposes, it's now possible
to define a G_RESOURCE_OVERLAYS environment variable as a
colon-separated list of substitutions to perform when looking up
GResources.

A substitution has the form

  "/org/gtk/libgtk=/home/desrt/gtk-overlay"

The part before the '=' is the resource subpath for which the overlay
applies.  The part after is a filesystem path which contains files and
subdirectories as you would like to be loaded as resources with the
equivalent names.

In the example above, if an application tried to load a resource with
the resource path '/org/gtk/libgtk/ui/gtkdialog.ui' then GResource would
check the filesystem path '/home/desrt/gtk-overlay/ui/gtkdialog.ui'.  If
a file was found there, it would be used instead.

Substitutions must start with a slash, and must not have a trailing
slash before the '='.  It is possible to overlay the location of a
single resource with an individual file.

https://bugzilla.gnome.org/show_bug.cgi?id=765668