Fix void-variable themes error at startup

If lexical-binding isn't enabled (with ';; -*- lexical-binding: t; -*-'
at the top of your private config), custom-set-theme! and
custom-theme-set-faces! throw this error. Since these macros are
commonly used in user configs, where I can't police lexical-binding,
I've refactored it to not rely on it.

core/autoload/themes.el

@@ -16,11 +16,10 @@
 THEME can be a single symbol or list thereof. If nil, apply these settings to
 all themes. It will apply to all themes once they are loaded."
   (declare (indent defun))
-  `(let* ((themes (doom-enlist (or ,theme 'user)))
-          (fn (gensym (format "doom--customize-%s-h-" (mapconcat #'symbol-name themes "-")))))
+  `(let ((fn (gensym "doom--customize-themes-h-")))
      (fset
       fn (lambda ()
-           (dolist (theme themes)
+           (dolist (theme (doom-enlist (or ,theme 'user)))
              (when (or (eq theme 'user)
                        (custom-theme-enabled-p theme))
                (apply #'custom-theme-set-faces 'user