@Cammy
Most CMS use template based systems. The templates are small bits of code in separate files that represent the actual structure of the pages and the content (from your database, image files etc) is inserted into it to generate the final HTML, which is then sent to your browser.
A complete set of templates usually comprises a theme. It ought to be possible to create one which uses old HTML attributes in the template markup to set colours and such and strip out as much of the CSS as possible. You don't need to rewrite the whole CMS, thankfully.