[eluser]kurucu[/eluser]
I would be surprised if any CMS forces you to stick to a single theme. But either way, in complete contrast ModX sees everything you can access as another 'resource' and so the output could be different themes, just as it could be a PDF, XML or a Flash movie. Therefore, you can make information as common or unique as you like. What's more, with contexts you can set up intranet stuff as common, and then override any setting you like in the local context.
I love CI to pieces, and am building a whole custom website with it, including forums, CMS, and two complex applications (which are private until I've finished). But, for what you want, I think CI would be overkill. Something like ModX would already implement security, listed content (e.g. news/blogs/galleries - in fact whatever you want), the editors, file management etc etc etc. Plus it lets you plug in and use any PHP you need to connect to other systems or display custom data. And you can put it on the page however you want - you don't have to comply to "boxes" or any of the other common CMS stuff.
In short, unless there is some specific functionality you need to write yourself, you'll just be reinventing the wheel. I am using CI because I want to use common and custom data across a variety of systems including some I am building myself. I don't have access to LDAP or anything else that might tie it together, plus I'm interested in writing and customising the software to be exactly what I want. While it might be fun, you don't have to force yourself down that route needlessly.
