They should go in wp-content/themes/style.css. Be aware that if you add them to the main theme’s style.css, this file will get overwritten if you update the theme (although the developer of this theme seems to have disappeared off the radar, so I don’t anticipate any updates ever).
To avoid this, you can either create a child theme, or just keep a copy of your styles.css file and make sure you copy any new stuff in to the style.css file after you’ve updated the theme.
Here’s my first ever WordPress site; I created it using Openstrap, and as you can see, it’s substantially modified, including div classes and all sorts of stuff: https://weareccfm.com/
– Paul
