Emacs is playing fancy on the latest version (since emacs 23) it has now support for transparency at least on Linux when you have a composited Windows Manager.
As explained on the Emacs wiki here everything is controlled by this frame parameter like this :
(set-frame-parameter (selected-frame) 'alpha '(85 50))
I have automated the thing to allow the transparency to increase or decrease when combined with the alt key just put this code somewhere in your $HOME/.emacs or $HOME/.emacs.d/init.el :
(defun my-increase-opacity() (interactive) (let ((increase (+ 10 (car (frame-parameter nil 'alpha))))) (if (> increase 99)(setq increase 99)) (set-frame-parameter (selected-frame) 'alpha (values increase 75))) ) (defun my-decrease-opacity() (interactive) (let ((decrease (- (car (frame-parameter nil 'alpha)) 10))) (if (< decrease 20)(setq decrease 20)) (set-frame-parameter (selected-frame) 'alpha (values decrease 75))) ) (global-set-key (kbd "M-
") 'my-increase-opacity) (global-set-key (kbd "M- ") 'my-decrease-opacity)