Make it easy for apps to have dark themes

Make it easy for apps to have dark themes

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