How to get background type of Emacs? e.g. 'light or 'dark

Question

How to get background type of Emacs? e.g. 'light or 'dark

You can define a face like this:

(defface moedict-type
  '((((class color) (background light))
     (:foreground "#ffffd7" :background "#525252"))
    (((class color) (background dark))
     (:foreground "#525252" :background "#c1c1c1")))
  "Face for type. ex: [動]、[名]")

And Emacs will select correct font face automatically by current background type. But I want to know how it do. (It is better if there is an build-in function in Emacs)

By the way, I try to seek in source code and found a function (frame-background-color), but its output is string like "#ffffff".