How to copy different template features into another template with multisite?

Hello,

I would like to create a Multisite NEtwork that has the same userbase for different domains and each domain has another template with a special feature. Like one template has a business directory and another has classified ads. These templates look completely different, but I would like to create an overall identical look that can still use those special features.

Does anyone know if this is possible? It would be enough to have the top section and main menu look the same and the rest (special features) remain like they are.

Any ideas?