ox-latex: Fix string/symbol engraved-preamble val

* lisp/ox-latex.el (org-latex-generate-engraved-preamble): The type of
engraved-theme could be a string or a symbol, depending on whether the
default value was used.  This can lead to type issue bugs.  It is better
to have the same type, so let's intern as needed to consistantly have
symbols.

lisp/ox-latex.el

@@ -1264,25 +1264,29 @@ which are given by `org-latex-engraved-preamble' and
          (engraved-preamble (plist-get info :latex-engraved-preamble))
          (engraved-theme (plist-get info :latex-engraved-theme))
          (engraved-themes
-          (cl-delete-duplicates
-           (org-element-map
-               (plist-get info :parse-tree)
-               '(src-block inline-src-block)
-             (lambda (src)
-               (plist-get
-                (org-export-read-attribute :attr_latex src)
-                :engraved-theme))
-             info)))
+          (mapcar
+           #'intern
+           (cl-delete-duplicates
+            (org-element-map
+                (plist-get info :parse-tree)
+                '(src-block inline-src-block)
+              (lambda (src)
+                (plist-get
+                 (org-export-read-attribute :attr_latex src)
+                 :engraved-theme))
+              info))))
          (gen-theme-spec
           (lambda (theme)
             (if (eq engrave-faces-latex-output-style 'preset)
-                (engrave-faces-latex-gen-preamble (when theme (intern theme)))
+                (engrave-faces-latex-gen-preamble theme)
               (engrave-faces-latex-gen-preamble-line
                'default
                (alist-get 'default
                           (if theme
                               (engrave-faces-get-theme (intern theme))
                             engrave-faces-current-preset-style)))))))
+    (when (stringp engraved-theme)
+      (setq engraved-theme (intern engraved-theme)))
     (when (string-match "^[ \t]*\\[FVEXTRA-SETUP\\][ \t]*\n?" engraved-preamble)
       (setq engraved-preamble
             (replace-match
@@ -1319,7 +1323,7 @@ which are given by `org-latex-engraved-preamble' and
                (lambda (theme)
                  (format
                   "\n\\newcommand{\\engravedtheme%s}{%%\n%s\n}"
-                  (replace-regexp-in-string "[^A-Za-z]" "" theme)
+                  (replace-regexp-in-string "[^A-Za-z]" "" (symbol-name theme))
                   (replace-regexp-in-string
                    "newcommand" "renewcommand"
                    (replace-regexp-in-string