(defun set-frame-alpha (alpha &optional frame)
  (when (and (>= 100 alpha)
             (<= frame-alpha-lower-limit alpha))
    (set-frame-parameter frame 'alpha alpha)))
;; (set-frame-alpha 80)

(defun increment-frame-alpha (&optional step frame)
  (interactive)
  (let* ((step (or step 10))
        (current-alpha (or (frame-parameter frame 'alpha) 100)))
    (set-frame-alpha (+ current-alpha step))))
;; (increment-frame-alpha)

(defun decrement-frame-alpha (&optional step frame)
  (interactive)
  (let ((step (or step 10))
        (current-alpha (or (frame-parameter frame 'alpha) 100)))
    (set-frame-alpha (- current-alpha step))))
;; (decrement-frame-alpha)