Load darktable.css instead of dakrtable.gtkrc

This is just a sample .css file with a dark background color to see if
loading worked.
Temporarily enable deprecated GTK/GDK symbols
Add -Wno-error=deprecated-declarations to FLAGS
GTK3: gtk_paint_box format has changed

Relevant Coccinelle patch:
@@
expression STYLE, W, STATE, ST, WG, DEL, STR, X, Y, WD, HT;
@@
gtk_paint_box(
STYLE,
-W,
+gdk_cairo_create(W),
STATE,
ST,
-DEL,
WG,
STR,
X,
Y,
WD,
HT)
GTK3: size_request is now get_preferred_(width,height)
GTK3: expose_event is now draw, second param is different
GTK3: GtkObject is gone

GtkObject has been removed in GTK+ 3. Its remaining functionality, the ::destroy signal, has been moved to GtkWidget. If you have non-widget classes that are directly derived from GtkObject, you have to make them derive from GInitiallyUnowned (or, if you don't need the floating functionality, GObject). If you have widgets that override the destroy class handler, you have to adjust your class_init function, since destroy is now a member of GtkWidgetClass.

See https://developer.gnome.org/gtk3/3.10/ch24s02.html#id-1.6.3.4.15
GTK3: gdk_devices_list() is gone
GTK3: gdk_device_get_core_pointer() is gone
GTK3: gtk_widget_set_extension_events() is gone
GTK3: GdkImage is gone
FIXME: is that correct?