"I can't seem to find a way to do that." - How hard have you tried? ;)
The RP has the extra data fields - so you can hang like a CSS name off the RP in the registration database. Further you can get to that RP data from the signin page - quoting the comment in AccountController:
// you can call AuthenticationHelper.GetRelyingPartyDetailsFromReturnUrl to get more information about the requested relying party
btw - IdentityServer's github repo has an issue tracker - you should use that for questions.