Thanks for the free publicity! I did that by using
background-attachment: fixed on those divs.
However, as I later found out, that effect is widely dependent on browser because it’s expensive to render. On mobile Chrome and Safari, for example, it doesn’t work; the background scrolls with the page as usual. But it works as intended on mobile Firefox. So a majority of the people reading this post won’t see the effect you mention. A hard lesson in UX design!
Apparently there are some workarounds, so I’ll be trying those soon.