init.el

(defun toggle-transparency ()
  "Toggles the transparency of the current frame"
  (interactive)
  (let ((alpha-value 85))
    (cond
     ((equal (frame-parameter nil 'alpha) nil) ; case 1
      (set-frame-parameter (selected-frame) 'alpha '(alpha-value alpha-value))) ; action 1 
     ((= alpha-value (car (frame-parameter nil 'alpha))) ; case 2
      (set-frame-parameter (selected-frame) 'alpha '(100 100))) ; action 2
     (t ; default case
      (set-frame-parameter (selected-frame) 'alpha '(alpha-value alpha-value))))) ; default action
  )