I came across the idea of implementing night mode using
mix-blend-mode: difference when I was preparing for my talk This World Mixed and Blended (these slides require color font support, i.e., FireFox / Safari). The original idea was only meant for a showcase and the browser support is currently limited.
Still, I implemented it to my dev blog and had a lot of fun. So this will be what this post is about. Meanwhile, as I thought more about it, it occurs to me now that this can be a legit preferred way night mode is to be done. And let me share a bit about the intuition behind that as well.