https://www.oracle.com/webfolder/ux/middleware/alta/index.html
https://developer.apple.com/design/human-interface-guidelines/ios/overview/themes/
Shopify’s Polaris is well documented and its impact is apparent throughout their product.
The Co-op design system has specific guidance for accessibility standards (and some excellent focus states!).
Slack’s media kit illustrates how they express their brand visually.
Mailchimp’s Content Style Guide and Branding Design defines guides or rules around writing content that help keep their voice consistent. Some tips they include: “Use active voice. Avoid passive voice.” “Avoid slang or jargon.” “Use positive language rather than negative language.”
Google’s Material Design lays out extremely flexible color and layout systems to enable app developers to create their own look that works well in the android ecosystem.