HTML/CSS to theme/templates

Hey guys I have a quick question. I’m using wordpress more as a CMS than for blogging. In fact I won’t even have a blog on the site, but I want full control over the design aspects of it which may change frequently.

So I need to convert a bunch of html/css files into a theme. I’m just wondering a little bit how this works. My main page is very different from the other pages (Which are different in themselves).

So how do I do this? As I understand I can design the look of each page with a separate template for each unique page design, is that true? And the theme contains the most important bits like the header, navigation and footer?