Use explicit menu item background if it's given under MSW.

An explicit item background colour specified in user code should override the
default theme background.

Closes #12652.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@66015 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775

samples/menu/menu.cpp

@@ -465,6 +465,8 @@ MyFrame::MyFrame()
 #if USE_LOG_WINDOW
     wxMenuItem *item = new wxMenuItem(fileMenu, Menu_File_ClearLog,
                                       wxT("Clear &log\tCtrl-L"));
+    item->SetBackgroundColour(*wxBLACK);
+    item->SetTextColour(*wxRED);
 #if wxUSE_OWNER_DRAWN || defined(__WXGTK__)
     item->SetBitmap(copy_xpm);
 #endif