custom-known-themes is a variable defined in `custom.el'.
Its value is
(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.