It's not down to the code style level, but Atlassian has the Atlassian Design Guidelines (ADG) and Atlassian UI (AUI).

ADG is the general patterns and UX stuff, AUI is a UI library which covers both CSS and JS components and implements the patterns in the ADG.

ADG: https://design.atlassian.com/latest/ AUI: https://docs.atlassian.com/aui/latest/