You may wish to modify some design elements and that may require using a custom css.

You can add custom css in Theme Options > Design > Custom CSS. The modifications you do here are NOT overwritten when you do a theme update.

• The theme offers a basic child theme in the theme pack. This allows you to make changes to your site without modifying the parent theme – so you can add custom css in style.css file from the child theme.  When a new version of the parent theme is released, after you update it on your site, all your modifications from the child theme are saved and it won’t be affected or lost.
  Help article for child theme install

To see css changes, you will need to clear cache.