This is an experiment...

I created a Raw HTML page. I published the page. I copied the source code from the browser. Back in Sandvox, I changed the page to Fill the Entire Page and pasted the browser copied HTML to replace the original Sandvox page.

Doing this I was able to edit the Site Menu and keep the CSS look of the page with minimal effort. It also means that the page can be edited from within Sandvox and so will not be over-written with each Publish All.

Neil