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.
If Minify css option is enabled each time you make changes with CSS, you will need to disable Minify css and js file option, make the change that you wish and after enable the option again. Help for this option: http://help.wpresidence.net/article/wpresidence-theme-options-minify-css-and-js-file/
Or you can use child theme – and add the changes directly in wpresidence-child-po/style.css file.
The theme offers a basic child theme in the theme pack. Help for child theme install: http://help.wpresidence.net/article/how-to-install-child-theme/