Your custom WordPress theme uses several open source frontend frameworks to simplify building a mobile responsive website. The primary frameworks are Bootstrap 4 and Font Awesome 5. So any CSS/HTML features within these frameworks should work on your website. We highly recommend familiarizing yourself with the bootstrap documentation.
On top of these frameworks are custom CSS classes that give a range of access to branding colors and other tools to maintain a consistent website. If you need help always remember that you can email firstname.lastname@example.org. Full Media support can always add additional custom CSS in the future.