Disable jquery.ui.theme.css only for front end?
I would use user_is_anonymous() function like this:

function yourtheme_css_alter(&amp;$css) { 
  if(user_is_anonymous()) {
    $exclude = array(
      'misc/ui/jquery.ui.core.css' =&gt; FALSE,
      'misc/ui/jquery.ui.theme.css' =&gt; FALSE,
    );
    $css = array_diff_key($css, $exclude);
  }
}