how can I make emacs use other css for orgmode publish

Use the :style parameter in your publishing project. For example:

(setq org-publish-project-alist
      '(("Homepage"
     :base-directory "~/git/homepage/"
     :base-extension "org"
     :publishing-directory "~/public_html/"
     :publishing-function org-publish-org-to-html
     :style "<link rel=\"stylesheet\" href=\"style.css\" type=\"text/css\" />")))

You can also define a per-file option like this:

#+STYLE: <link rel="stylesheet" type="text/css" href="stylesheet.css" />

Do C-h v org-export-html-style TAB TAB to list various options related to the style for HTML export.