patch 8.2.2111: GTK: menu background is the same color as the main window

Problem:    GTK: Menu background is the same color as the main window.
Solution:   Fix white space around the test in another way. (closes #7437,
            closes #7427)
diff --git a/src/gui_gtk_x11.c b/src/gui_gtk_x11.c
index e06bfd7..22788e8 100644
--- a/src/gui_gtk_x11.c
+++ b/src/gui_gtk_x11.c
@@ -3816,11 +3816,11 @@
 #if !GTK_CHECK_VERSION(3,0,0)
     gtk_widget_set_events(gui.formwin, GDK_EXPOSURE_MASK);
 #endif
+#if GTK_CHECK_VERSION(3,22,2)
+    gtk_widget_set_name(gui.formwin, "vim-gtk-form");
+#endif
 
     gui.drawarea = gtk_drawing_area_new();
-#if GTK_CHECK_VERSION(3,22,2)
-    gtk_widget_set_name(gui.drawarea, "vim-gui-drawarea");
-#endif
 #if GTK_CHECK_VERSION(3,0,0)
     gui.surface = NULL;
     gui.by_signal = FALSE;
@@ -4031,18 +4031,18 @@
     void
 gui_mch_new_colors(void)
 {
-    if (gui.drawarea != NULL && gtk_widget_get_window(gui.drawarea) != NULL)
+    if (gui.formwin != NULL && gtk_widget_get_window(gui.formwin) != NULL)
     {
 #if !GTK_CHECK_VERSION(3,22,2)
-	GdkWindow * const da_win = gtk_widget_get_window(gui.drawarea);
+	GdkWindow * const da_win = gtk_widget_get_window(gui.formwin);
 #endif
 
 #if GTK_CHECK_VERSION(3,22,2)
 	GtkStyleContext * const context
-	    = gtk_widget_get_style_context(gui.mainwin);
+	    = gtk_widget_get_style_context(gui.formwin);
 	GtkCssProvider * const provider = gtk_css_provider_new();
 	gchar * const css = g_strdup_printf(
-		"widget#vim-gui-drawarea, #vim-main-window {\n"
+		"widget#vim-gtk-form {\n"
 		"  background-color: #%.2lx%.2lx%.2lx;\n"
 		"}\n",
 		 (gui.back_pixel >> 16) & 0xff,