frames: Bump priority of style providers

Since GTK+ commit b1ad5c8abc2c, GtkSetting's CSS provider uses a
priority of GTK_STYLE_PROVIDER_PRIORITY_SETTINGS, which means it
will overwrite the ones we create ourselves.
Bump the priority to fix dark window decorations.

https://bugzilla.gnome.org/show_bug.cgi?id=688182

src/ui/frames.c

@@ -209,7 +209,7 @@ create_style_context (MetaFrames  *frames,
       provider = gtk_css_provider_get_named (theme_name, variant);
       gtk_style_context_add_provider (style,
                                       GTK_STYLE_PROVIDER (provider),
-                                      GTK_STYLE_PROVIDER_PRIORITY_THEME);
+                                      GTK_STYLE_PROVIDER_PRIORITY_SETTINGS);
     }
 
   g_free (theme_name);