Spent ages trying to find out why login, register and page not found single pages weren't using my view.php file and kept using the concrete5 wrapper.

The answer is to change the config/site_theme_paths.php for these specific pages.

Thank you jordanlev for this post, it worked:

re: Superfish not holding styling from one single page to another
Sep 30, 2010 at 3:41 PM
Did you enable your custom theme for those single pages? If they're system pages (like Profile or Avatar or Register or Login), you need to explicitly tell C5 to use your theme instead -- in the config/site_theme_paths.php file.

Putting answer here to save next person time hunting for it.