4 sites... 3 menus... what's the best approach

[studiodpi]

Hello... looking for some feedback for a site I originally was going to design using just CSS/HTML but now want to put it into WP using a theme to best compliment the set up

so there are essentially 4 sites, depending on which section you're in the bg colour would change to indicate the space you're visiting and the navigation would change to reflect the content as well. (see attachment)

I'm just not sure if the best approach is 4 sites using the same theme but changing each of the bg & menus for that section/mini site or if editing the CSS to recognize each mini site would be easier for the client to manage. I'm not a CSS wiz.. I can find my way around and make some changes but from scratch I'd be scratching my head for awhile at this point

Feedback GREATLY appreciated

Cheers,
Malinda