Again, this sounds like you want to use CSS customizations and plugin outlets. Without a more detailed description or mockup of what you want to do, I’m afraid I cannot provide any additional help.
While this is quite vague, it sounds like what you want to build would be insecure. Depending on what exactly you want to accomplish, SSO, the Data Explorer or a custom plugin might help you, but we need a detailed description of what you want to have to help you.