Sticky navigation below page header

Hi guys,

Thanks for an excellent theme. I've been playing around with it – and got stuck with something.

I'm using a page header (background image and content, full-screen) and I want the primary menu positioned below it and sticky. But I can't figure out how to do this for the life of me.

Here's a screenshot of what I have at present, with the menu above the page header:

Thanks.