frame-terminal-default-bg-mode

frame-terminal-default-bg-mode is a compiled Lisp function in `frame.el'.

(frame-terminal-default-bg-mode FRAME)

Return the default background mode of FRAME.
This checks the `frame-background-mode' variable, the X resource
named "backgroundMode" (if FRAME is an X frame), and finally
the `background-mode' terminal parameter.