Sorry, missed that part. For free, it’s a little more complicated. You’ll need to setup a child theme:
https://startupwp.com/topic/child-theming/
But, in pro as already indicated, you can use a convenient Custom CSS area within the theme options without all the extra work.
Either way, you’ll want to use one of these methods instead of manually editing the stylesheet for example, otherwise those edits will be overwritten and lost upon upgrading (everytime).