showdesktop: Port gtk_widget_get_style to GtkStyleContext
showdesktop: Port gtk_widget_get_style to GtkStyleContext

Signed-off-by: Germán Póo-Caamaño <679831b636f7f6776b5d7665ebec4c0074d994a9@gnome.org>
GNOME/gnome-panel,GNOME/gnome-panel