custom-known-themes
#emacs28.2 #custom #VARIABLE
https://github.com/emacs-mirror/emacs/blob/emacs-28.2/lisp/custom.el#L863-L869
code: custom.el
(defvar custom-known-themes '(user changed)
"Themes that have been defined with `deftheme'.
The default value is the list (user changed). The theme `changed'
contains the settings before custom themes are applied. The theme
`user' contains all the settings the user customized and saved.
Additional themes declared with the `deftheme' macro will be added
to the front of this list.")