They first starting appearing after Fable in my memory. The first HTML docs I generated with Fable had that cream serif with orange highlight look. I thought it looked awesome until I saw it on every other vibe-coded website...
Every dark mode one with the rounded boxes with the weird use of border one side to highlight but it curves around the edge and also the tiny capitalized monospaced micro-type everywhere.
(Randomly, I love the Solarized colour schemes, so anything similar is great, vibe-coded or not.)