I would not think of CSS as necessarily being dynamic.
One factor is size, another (as dcheney pointed out) is how often it changes. On a small site it is simple enough to make occassional changes on many pages by opening them up in an editor (wysiwyg or html, I prefer editing html) and cutting and pasting. I maintain one site for a local organization. Every couple of weeks I get a little more information about a future meeting, change in committee assignments, etc. I can edit the affected pages and upload them in just a few minutes.
I have also built larger sites using a database on my computer with a VBA program that builds the static pages, and I upload them. I would consider the pages static although they are created by a database.