For hosted or on-prem instances you can map themes to specific SPACES and URL maps: This from the Admin Console / System / Settings / Themes:
Custom Theme Maps:
Themes can be mapped to various application data such as Spaces and URL patterns. This enables a single Jive instance to display multiple skins throughout the application. Theme maps can be applied without a server restart and can be modified anytime.
So you can customize a separate theme and map that specific theme to a set of Spaces or Groups. I would go slow though and not create a world of various different themes as any upgrade will lengthen your testing/qa cycle, not to mention your higher admin effort.