Workaround an issue with getting theme colours

I see that GTK+ 3 is no longer something to be used.

liblogdiag/ld-category-symbol-view.c

@@ -350,6 +350,9 @@ on_draw (GtkWidget *widget, cairo_t *cr)
 		return FALSE;
 
 	context = gtk_widget_get_style_context (widget);
+	// FIXME: there have to be better means (though I can't find them)
+	gtk_style_context_add_class (context, "gtkstyle-fallback");
+
 	gtk_style_context_get_background_color (context,
 		GTK_STATE_FLAG_NORMAL, &color);
 	gdk_cairo_set_source_rgba (cr, &color);