You can do it with 'External Learners.' The default configuration for an external learner group includes branding imagery - which as i recall then shows up in the upper right corner when the external learners login. (Its fairly simple - just the name of org and icon)
For the sake of clarity, you can also define separate catalogs - and permit access based on any major parameter / filter - individuals, groups etc. to any or all of those catalogs (collections of courses) and of course can include courses in multiple catalogs. That however is not independently 'skinnable - brandable at this time.' If this option is the sort of branding / skinning you are looking for - I'd encourage you to add detail here or in an email with subject Enhancement Request to CaptivateHelp@adobe.com - skinning / branding is a major feature update scheduled for June - so now is a very good time for such requests.
--Allen