Add CSS links for the current page and for the plugins

def _css_helper(self):
        """ Add CSS links for the current page and for the plugins """
        entries = [entry for entry in self._plugin_manager.call_hook("css") if entry is not None]
        # Load javascript for the current page
        entries += self._get_ctx()["css"]
        entries = ["<link href='" + entry + "' rel='stylesheet'>" for entry in entries]
        return "\n".join(entries)