(defun remx/set-opacity (value &optional frame)
"Set opacity to VALUE for FRAME.
VALUE is a pair of active and inactive opacity values
between 0-100 (where 0 is transparent and 100 is opaque).
If FRAME is nil, it defaults to the selected frame."
(interactive "nOpacity (0 is transparent - 100 is opaque): ")
(set-frame-parameter frame 'alpha (cons value value)))