if you want Emacs to examine the brightness for you.
If you change this without using customize, you should use
-`frame-set-background-mode' to update existing frames."
+`frame-set-background-mode' to update existing frames;
+e.g. (mapc 'frame-set-background-mode (frame-list))."
:group 'faces
:set #'(lambda (var value)
(set-default var value)