The Complete Computing Environment

(defun cce/org-inline-css-hook (exporter)
  "Insert custom inline css"
  (when (eq exporter 'html)
    (let* ((dir (ignore-errors (file-name-directory (buffer-file-name))))
           (path (concat dir "style.css"))
           (homestyle (or (null dir) (null (file-exists-p path))))
           (final (if homestyle "~/org/style.css" path))) ;; <- set your own style file path
      (setq org-html-head-include-default-style nil)
      (setq org-html-head (concat
                           "<style type=\"text/css\">\n"
                           "<!--/*--><![CDATA[/*><!--*/\n"
                           (with-temp-buffer
                             (insert-file-contents final)
                             (buffer-string))
                           "/*]]>*/-->\n"
                           "</style>\n")))))

(add-hook 'org-export-before-processing-hook 'cce/org-inline-css-hook)