(defvar modus-themes-after-load-theme-hook nil
  "Hook that runs after the `modus-themes-toggle' routines.")

(defun modus-themes--light ()
  "Load `modus-operandi' and disable `modus-vivendi'."
  (disable-theme 'modus-vivendi)
  (load-theme 'modus-operandi)
  (run-hooks 'modus-themes-after-load-theme-hook))

(defun modus-themes--dark ()
  "Load `modus-vivendi' and disable `modus-operandi'."
  (disable-theme 'modus-operandi)
  (load-theme 'modus-vivendi)
  (run-hooks 'modus-themes-after-load-theme-hook))

(defun modus-themes--load-prompt ()
  "Helper for `modus-themes-toggle'."
  (let ((theme
         (intern
          (completing-read "Load Modus theme (will disable all others): "
                           '(modus-operandi modus-vivendi) nil t))))
    (mapc #'disable-theme custom-enabled-themes)
    (pcase theme
      ('modus-operandi (modus-themes--light))
      ('modus-vivendi (modus-themes--dark)))))

;;;###autoload
(defun modus-themes-toggle ()
  "Toggle between `modus-operandi' and `modus-vivendi' themes.
Also runs `modus-themes-after-load-theme-hook' by virtue of
calling the internal `modus-themes--light' and
`modus-themes--dark' functions."
  (interactive)
  (pcase (car custom-enabled-themes)
    ('modus-operandi (modus-themes--dark))
    ('modus-vivendi (modus-themes--light))
    (_ (modus-themes--load-prompt))))