SetOptions[$FrontEndSession, Background -> Yellow]
(* OK: the background is now yellow *)

SetOptions[EvaluationNotebook[], Background -> LightBlue]
(* OK: the background is now LightBlue*)

SetOptions[EvaluationNotebook[], Background -> Inherited]
(* OK: the inherited background color is now yellow -> $FrontEndSession one *)

SetOptions[EvaluationNotebook[], Background -> LightBlue]

SetOptions[EvaluationNotebook[],
NotebookEventActions -> {{"KeyDown", "b"} :> (SetOptions[EvaluationNotebook[],
  Background -> Inherited])}]

CurrentValue[EvaluationNotebook[], Background]