frame-background-mode

frame-background-mode is a variable defined in `frame.el'.
Its value is

nil

Documentation:
The brightness of the background.
Set this to the symbol `dark' if your background color is dark,
`light' if your background is light, or nil (automatic by default)
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;
e.g. (mapc 'frame-set-background-mode (frame-list)).

You can customize this variable.