bug#29443: Document how to set themes depending on a condition

["積丹尼 Dan Jacobson" <jidanni@jidanni.org>]

In (info "(emacs) Custom Themes") after

    Setting or saving Custom themes actually works by customizing the
    variable ‘custom-enabled-themes’.  The value of this variable is a list
    of Custom theme names (as Lisp symbols, e.g., ‘tango’).  Instead of
    using the ‘*Custom Themes*’ buffer to set ‘custom-enabled-themes’, you
    can customize the variable using the usual customization interface,
    e.g., with ‘M-x customize-option’.

please add:

    Here's how to set a theme depending on a condition:

    (custom-set-variables
     ;; custom-set-variables was added by Custom.
     ;; If you edit it by hand, you could mess it up, so be careful.
     ;; Your init file should contain only one such instance.
     ;; If there is more than one, they won't work right.
     (if(/= 0(user-uid)) '(custom-enabled-themes (quote (wheatgrass)))))

    In the above we have added a condition to what Custom has put into the
    customization file we found on our disk.

    We set the wheatgrass theme if we are not root.

    One could go further and set different themes for different seasons. The
    sky's the limit!