I really like Perch for this very reason. It can handle my nav, CSS, and content if I want it to, but I don't have to put it in the CMS. Any page can have a completely different design that departs from other designs, but you can still make it really easy for non-developers to update images, copy, etc. If you already have a site that needs a CMS, Perch is also really good in that case. I have a few Perch licenses for myself and some customers.

https://grabaperch.com