markdown-edit-mode.el

;;; markdown-edit-mode.el --- provide markdown-editing environment

(make-local-variable
 (defvar markdown-prev-theme spacemacs--cur-theme))

(defcustom markdown-edit-mode-hook nil
  "The hook to run when markdown-edit-mode is toggled."
  :type 'hook)
(defcustom markdown-edit-theme 'leuven
  "Theme to load for markdown environment.")

;;;###autoload
(define-minor-mode markdown-edit-mode
  "Toggle markdown-edit-mode.
With ARG, turn markdown-edit-mode on if ARG is positive."
  :global nil
  :lighter " me"
  (if markdown-edit-mode
      (progn
        (switch-to-buffer-other-frame (current-buffer))
        (load-theme markdown-edit-theme t)
        (spacemacs/toggle-line-numbers-off)
        (spacemacs/disable-hl-line-mode)
        (set-window-margins (selected-window) 20 20)
        (set-face-attribute 'default (selected-frame) :height 140)
        (set-frame-size (selected-frame) 125 65))
    (progn
      (set-window-margins (selected-window) 0 0)
      (spacemacs/toggle-line-numbers-on)
      (delete-frame)
      (helm-themes--load-theme (symbol-name markdown-prev-theme)))))

(provide 'markdown-edit-mode)

Made a couple of changes in the fork: https://gist.github.com/al3xandru/86375ad91ec03ad40d76