register_theme_directory somehow “fails” when folder is outside of WP-Folder

This is a simple skeleton for WordPress, that makes it possible to have just “themes” and “plugins” under version control:

https://github.com/sourcerer-mike/WordPress-GIT-Skeleton

When doing register_theme_directory(__DIR__ . '/themes'); with a folder that is outside of WP, the theme is shown in the backend and can be activated.

But when viewing the front end the page stays blank.

I guess it’s up to the folder being one level above and unreachable for the client. But the server can access it, so I think I’ve done something wrong.

Is there any solution to have themes outside of WP but still working? (otherwise register_theme_directory is kind of useless)

Or is there a better solution to have those two outside?
(I also need to take care of not so experienced developer here)

My folder structure is like this:

├── plugins
├── public (document root of apache)
│   ├── wp-admin
│   ├── wp-content
│   │   ├── plugins
│   │   └── themes
│   │       ├── twentyeleven
│   │       ├── twentyten
│   │       ├── twentythirteen
│   │       └── twentytwelve
│   │       └── twentyfourteen
│   └── wp-includes
├── themes
│   └── foo
└── uploads

As soon as I change to theme “foo” the frontend stays white and empty (no single output).