Show HN: A Website Scavenger Hunt

Hello HN! I made a scavenger hunt on my website in which you need to find the different background images I use.

There are 2,000 pages and four different backgrounds to find. How fast can you find them?

I am sharing this here since a few people have already tried this and had fun. I hope it brings some joy to your day!

(Resist sharing the answers, but the details of the technical means by which you find them would be interesting to know.)