shell: Don't use magic constant for CSS provider priority

Signed-off-by: Guido Günther <guido.gunther@puri.sm>

src/shell.c

@@ -258,7 +258,8 @@ css_setup (PhoshShell *self)
 
   gtk_css_provider_load_from_resource (provider, "/sm/puri/phosh/style.css");
   gtk_style_context_add_provider_for_screen (gdk_screen_get_default (),
-                                             GTK_STYLE_PROVIDER (provider), 600);
+                                             GTK_STYLE_PROVIDER (provider),
+                                             GTK_STYLE_PROVIDER_PRIORITY_APPLICATION);
 }