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Parallax scrolling border?

I’m very much a noobie here. I understand basic html and css principles, but my background is illustration and animation. I’ve been playing around with creating simple scrolling ‘stories’ in html using parallax and other ‘on scroll’ animations. I was using Adobe Muse for a while, but switched to Webflow when Adobe dropped Muse. Webflow seems incredibly powerful and it might be just the thing for what I’m trying to achieve, but I’m struggling wrapping my head around some of the concepts of laying out page elements ‘on top of’ one another.
For example, right now I’d like to add a simple border to the left and right edge of the page, that can parallax scroll in the ‘foreground’. Imagine some leaves peeking in from the sides, and as you scroll down the page (which may be many screens high), the fringe of leaves move faster than the rest of the page content.
I’ve made a graphic, which I’ve used as the background of a divblock and made it tile vertically… but I can’t work out where to put this divblock in the navigator stack to ensure it shows over the whole of the page, rather than just one section. Any advice gratefully received,
here’s a link,