For the themes which installed through elpa, add the following code to auto include the theme path in the `pkg-autoloads.el` file under elpa.

auto-load-for-theme.el

;;;###autoload
(and load-file-name
     (boundp 'custom-theme-load-path)
     (add-to-list 'custom-theme-load-path
                  (file-name-as-directory
                   (file-name-directory load-file-name))))

(provide-theme 'theme-name)