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
`custom-theme-directory'.
- 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
name.