Some types of applications would benefit from having "dark" themes,
usually black backgrounds, such as:
* Movie players
* Photo management and display applications

To make it easy for those applications to prefer a dark theme,
we're adding the "gtk-application-prefer-dark-theme" GtkSetting, which
will make the theme loading code automatically look for a "gtkrc-dark"
file in the same directory you would usually find a gtkrc file.
the same name and a "-dark" suffix.

If no "-dark" gtkrc variant is available, the normal gtkrc will
be used.

https://bugzilla.gnome.org/show_bug.cgi?id=617955