custom-theme-directory is a variable defined in `custom.el'.
Its value is

Default user directory for storing custom theme files.
The command `customize-create-theme' writes theme files into this
directory. By default, Emacs searches for custom themes in this
directory first---see `custom-theme-load-path'.

You can customize this variable.

This variable was introduced, or its default value was changed, in version 22.1 of Emacs.