Elisp newbie string to symbol help

[Thomas Ingram <taingram@mtu.edu>]

I'm looking for some help with a simple function I am trying to write. 
I'm a long time Emacs user but I have little experience with Emacs lisp. 
The issue I am having is with the load-theme lines.

(defun my/theme-toggle ()
   "Toggle between light to dark theme."
   (interactive)
   (require 'custom)
   (let* ((current-theme (symbol-name (car custom-enabled-themes)))
      (theme (substring current-theme 0
                (- (length current-theme)
                   ; Probably a better way to do this with regex?
                   (if (string-suffix-p "dark" current-theme)
                   (length "-dark")
                 (length "-light"))))))
   (if (string-suffix-p "dark" current-theme)
       (load-theme (concat theme "-light"))
     (load-theme (concat theme "-dark"))))
   ;; (spaceline-emacs-theme)
   (start-process "toggle Gtk theme" nil "~/bin/change-theme" "toggle"))

I get the message "load-theme: Invalid theme name ‘spacemacs-dark’" 
which doesn't make sense as that is a theme I have installed.  I've 
tried quoting those lines but then it doesn't evaluate. How can I 
properly pass this concatenated string to load-theme?

I vaguely understand that it is expecting a symbol not a string and I've 
tried passing the string to make-symbol but that just changed the error 
to "Undefined Custom theme spacemacs-dark"

Any help is appreciated.

Thanks,

Thomas Ingram