We recommend to never modify the theme’s style.css file, as you will lose any changes you make to it at the next update (or you are forced to never update, losing out on all the bugfixes, improvements, features and new bugs added 🙂 )
You could of course use a dedicated custom CSS plugin or even a child theme, but for short / simple CSS customizations we have created a special input field right in theme’s settings page. You’ll find it by going to Appearance > Theme Settings, then expanding the Miscellaneous Section by clicking on it. The Custom CSS field is a bit further down. (View screenshot)
If you don’t know what CSS is, we recommend starting here.