Q:

How do i make my gtk app use the dark theme?

Some apps on the gnome desktop like the image viewer use a dark variation of the theme.
What code is required to make my own gtk program use this dark variation of the theme?

A:

Use the gtk-application-prefer-dark-theme setting. This ought to do it:
g_object_set(gtk_settings_get_default(),
    "gtk-application-prefer-dark-theme", TRUE,
    NULL);

You may need to make sure you do it before you create any widgets.

