Update style definitions with the Cantarell>Inter switch

While the switch to Inter hasn't happened yet, it would be nice to roll out the change across various projects and websites.

• Cantarell mini website.
• GNOME Brand. Website + Brand Book & templates.
• Handbook
• HIG
• Developer
• Welcome
• Apps
• Nightly
• This Week in GNOME
• Release
• GSOC
• Orca