Insert the readtheorg.css links wth html export advice

It removes the need to clutter Spacemacs documentation with
Also it reduces the amount of bugs, because it's hard to get the path right.

core/core-documentation.el

@@ -13,6 +13,7 @@
 (require 'ox-publish)
 (require 's)
 (require 'dash)
+(require 'f)
 
 (defvar spacemacs--category-names
   '(("config-files" . "Configuration files")
@@ -85,11 +86,49 @@
         (format "%s\n%s%s" beginning-of-content toc-string rest-of-content)
       content)))
 
+(defun spacemacs//add-org-meta-readtheorg-css (filename)
+  (let* ((head-css-extra-readtheorg-head (concat
+                                          "#+HTML_HEAD_EXTRA:"
+                                          "<link rel=\"stylesheet\" "
+                                          "type=\"text/css\" "
+                                          "href=\""))
+         (head-css-extra-readtheorg-tail "css/readtheorg.css\" />\n"))
+    (progn (goto-char (point-min))
+           (delete-matching-lines "\\+HTML_HEAD_EXTRA\\:.*\\/css\\/readtheorg\\.css")
+           (goto-char (point-min))
+           (if (search-forward "#+TITLE:" nil t nil)
+               (beginning-of-line 2)
+             (error "Can't find #+TITLE:"))
+           (insert (concat head-css-extra-readtheorg-head
+                           (f-relative user-emacs-directory
+                                       (file-name-directory filename))
+                           head-css-extra-readtheorg-tail)))))
+
+(defun spacemacs//pub-doc-html-advice (origfunc &rest args)
+  (save-current-buffer
+    (save-excursion
+      (let* ((filename (car (nthcdr 1 args)))
+             (visitingp (find-buffer-visiting filename)))
+        ;; Temporary "unvisit" the visited org files.
+        (when visitingp (with-current-buffer visitingp (setq buffer-file-name nil)))
+        (with-temp-buffer
+          (save-match-data
+            (insert-file-contents filename t)
+            (spacemacs//add-org-meta-readtheorg-css filename)
+
+
+
+            (apply origfunc args)
+            (not-modified)))
+        ;; Restore `buffer-file-name' for the buffers that previously visited the org files.
+        (when visitingp (with-current-buffer visitingp (setq buffer-file-name filename)))))))
+
 (defun spacemacs/publish-doc ()
   "Publish the documentation to doc/export/."
   (interactive)
   (advice-add 'org-html-toc :filter-return #'spacemacs//format-toc)
   (advice-add 'org-html-template :filter-return #'spacemacs//format-content)
+  (advice-add 'org-html-publish-to-html :around #'spacemacs//pub-doc-html-advice)
   (let* ((header
           "<link rel=\"stylesheet\" type=\"text/css\"
                  href=\"http://www.pirilampo.org/styles/readtheorg/css/htmlize.css\"/>
@@ -144,6 +183,7 @@
              :publishing-function org-publish-attachment))))
     (org-publish-project "spacemacs"))
   (advice-remove 'org-html-toc #'spacemacs//format-toc)
-  (advice-remove 'org-html-template #'spacemacs//format-content))
+  (advice-remove 'org-html-template #'spacemacs//format-content)
+  (advice-remove 'org-html-publish-to-html #'spacemacs//pub-doc-html-advice))
 
 (provide 'core-documentation)