Use dark theme if available [$5]

[Erasmo Marín]

Like all multimedia apps are doing, pantheon-photos should use dark theme.

[Jacob Parker (jacobparker1992)]

To save looking up in the docs, it should be the following:
Gtk.Settings.get_default ().gtk_application_prefer_dark_theme = true;

[Erasmo Marín (erasmo-marin)]

That's not enough I think. I have tested doing that and the result is not good, it need more fixes, like using the background color of the theme instead of a custom color in the library page, but doing that requires discussion.

[Maxim Taranov (png2378)]

https://www.bountysource.com/issues/1441041-use-dark-theme-if-available

[Cassidy James Blaede (cassidyjames)]

Fixed in https://github.com/elementary/photos/pull/284