customize-themes doesn't allow backquote or :inherit

[Yavuz <justloop@gmail.com>]

Hello,
I have been trying out the customize-themes feature, and converting a
color-theme that I have to the new format. The problem I have is how to use
color variables instead of "#RRGGBB" type constants to ease design and
maintenance of themes. Initially I thought I could use something like:

(deftheme mytheme)
...
(defvar nice-color "#123456")
...
(custom-theme-set-faces 'mytheme
  `(default ((t (:foreground ,nice-color))))
  ...)
(provide-theme 'mytheme)

However, it seems that the load-theme function in custom.el has some code that
checks for the presence of 'quote and throws a 'Unsafe expression in theme
settings' error if I try to use ` instead of '.

Being an elisp newbie, my attempts to bypass is with various combinations of
eval, apply, and `' ended in failure.

Is there a way to use variables in custom-theme-set-faces? Or is that check
really need to be that strict? Maybe backquote would be acceptable as well?

Thanks in advance,
Yavuz