HI Stian,
Adding SSO zones just to address the theming issue looks a bit overkill to me as it will eventually come down to doing some theming at a level below the realm. I was going on the basis that if theming is not set at a client level, then it will default to the realm level theming which is basically your SSO enabled zone.
Also my other point was with regard to SaaS based applications where we have a backoffice system which is themed as per our SaaS product but the consumer facing front end needs to be themed to be aligned with the customer's web site. In this case, we cannot go with what KeyCloak has at present. What I am doing is as suggested by Bill sometime back, adding "if/else" statements into the freemarker templates and based on the client id loading different freemarker templates which is not ideal but does the job.
In any case, since what we are discussing is in general edge cases, Therefore instead of complicating the core KeyCloak platform, why don't you just expose the various links/flows that is currently available in the login process (forgot password/reset credentials, required actions (update password, verify email, configure OTP, etc.), user account mgmt, registration, social login etc. Then we are still using the core of keycloak but for the frontend themes/UI, we use our own.
I also haven't explored the Login SPI which as per the KeyCloak docs which says "The Login SPI allows implementing the login forms using whatever web framework or templating engine you want". Wonder if this will give us what we are after.
Cheers
Travis