Overwrite jquery.ui.slider.min.css generated by jQuery Update module
The solution is to insert in your themes template.php the following:

function mytheme_css_alter(&amp;$css) {
  if (isset($css['misc/ui/jquery.ui.theme.css'])) {
    $css['sites/default/modules/jquery_update/replace/ui/themes/base/jquery.ui.theme.css']['data'] = drupal_get_path('theme', 'mytheme') . '/mytheme_jquery_design/jquery.ui.theme.css';
  }
}