Revert "Bind the themes to the livecycle of the screen"

This reverts commit 1f5dea9eba,
since it was causeing noticable behaviour changes.

Previously, GTK_DATA_PREFIX=/ ./gtk3-demo would start
gtk3-demo with the Raleigh theme. With that change, it
was starting with no theme at all (i.e. all black).

gtk/gtkcssprovider.h

@@ -86,7 +86,8 @@ gboolean         gtk_css_provider_load_from_path (GtkCssProvider  *css_provider,
                                                   const gchar     *path,
                                                   GError         **error);
 
-GtkCssProvider * gtk_css_provider_get_default    (void);
+GtkCssProvider * gtk_css_provider_get_default (void);
+
 GtkCssProvider * gtk_css_provider_get_named (const gchar *name,
                                              const gchar *variant);