Get all directories under a certain one

custom-theme-dirs.el

;; add all theme directories to the `custom-theme-load-path`
;; directory automatically so `load-theme` can find them.
(let ((base "/home/user/.emacs.d/vendor/themes"))
  (add-to-list 'custom-theme-load-path base)
  (dolist (f (directory-files base))
    (let ((name (concat base "/" f)))
      (when (and (file-directory-p name) 
                 (not (equal f ".."))
                 (not (equal f ".")))
        (add-to-list 'custom-theme-load-path name)))))