start-dark.vala

// compile with: valac --pkg libwnck-3.0 -X -DWNCK_I_KNOW_THIS_IS_UNSTABLE --pkg x11 start-dark.vala

int main(string[] args)
{
    Gtk.init(ref args);

    if (args.length < 2)
        error("Please define a program");

    Pid pid = -1;
    try {
    Process.spawn_async(null, args[1:args.length - 1], Environ.get(), SpawnFlags.DO_NOT_REAP_CHILD, null, out pid);
    } catch (Error e) { error(e.message); }

    var screen = Wnck.Screen.get_default();
    screen.force_update();

    var dpy = new X.Display(null);
    var theme_variant = dpy.intern_atom("_GTK_THEME_VARIANT", false);
    var utf8_string = dpy.intern_atom("UTF8_STRING", false);

    Idle.add(() => {
        unowned List<Wnck.Window> windows = screen.get_windows();
        foreach (var window in windows) {
            if (window.get_pid() == pid) {
                dpy.change_property(window.get_xid(), theme_variant, utf8_string, 8, X.PropMode.Replace, (uchar[])"dark", 4);
                Gtk.main_quit();
                return false;
            }
        }
        return true;
    });

    Gtk.main();
    return 0;
}