Think I've got the fixed menu working now, thanks to a stack overflow answer I'm using box-shadow to draw outside the box of the element, so I can keep it at 8px from the top (standard body margin) and still make those 8px opaque.

I wonder when CSS is going to get references, so I can do background-color: ref(body.background-color); (yes, I could do something similar with variables, but I don't want to have to specify everything, I want to use whatever the user (browser) thinks is default.

Ah, well.