Static website serves you everything you need as a one HTML file. You can save the source code and open it offline and there will be no difference.

As such any templating is out of the question if that is done via request from the client.

How I manage my static website is with a build script. I write my content in markdown with a option for custom header and then I have a python script that churns out pure HTML with everything embedded into it which I can then host on github pages.