custom-check-theme
#custom
#emacs28.2
#SUBST
THEME -> NIL
theme が
custom-known-themes
に含まれるか確認する関数。ない場合はエラーになり、戻り値はnilになる
https://github.com/emacs-mirror/emacs/blob/emacs-28.2/lisp/custom.el#L875-L878
code: custom.el
(defsubst custom-check-theme (theme)
"Check whether THEME is valid, and signal an error if it is not."
(unless (custom-theme-p theme)
(error "Unknown theme `%s'" theme)))
defsubst
unless
custom-theme-p
error