Yet Another Overrides Question

[afo]

If you don't see a /classic folder, there is no override available in that area. On the rare occasions I find it necessary to make changes to core files, I create an override file anyway and keep a copy of my modified file in it. That way when I upgrade/update, I can find the changes I made and duplicate them.

You shouldn't be creating a custom folder in the /template_default folder. You should create it at the level above that, so it would be includes/templates/piccadilly_posh/css/stylesheet.css

includes/languages/piccadilly_posh/english.php and /includes/languages/english/piccadilly_posh>button_names.php apeear to be created corroctly.

What are you trying to do that requires changing files where no override is available?