custom-theme-load-path is a variable defined in `custom.el'.
Its value is
(custom-theme-directory t)

List of directories to search for custom theme files.
When loading custom themes (e.g. in `customize-themes' and
`load-theme'), Emacs searches for theme files in the specified
order. Each element in the list should be one of the following:
- the symbol `custom-theme-directory', meaning the value of
- the symbol t, meaning the built-in theme directory (a directory
named "themes" in `data-directory').
- a directory name (a string).

Each theme file is named THEME-theme.el, where THEME is the theme

You can customize this variable.

This variable was introduced, or its default value was changed, in version 24.1 of Emacs.