Hi!
We would never recommend editing core plugin code: as soon as you next update the plugin, you would lose the modification 🙁
If you’re unhappy with the idea of adding it via your theme’s custom CSS field, how about creating a new stylesheet within your theme:
wp-content/themes/YOUR_THEME/tribe-events/tribe-events.css
You can learn more about safely customizing styles here and do also note you could even do this from within a child theme, if you preferred.
Does that help?