Hey Rob!
topic-list.scss is part of Discourse core, that’s why you’re not seeing it. discourse/app/assets/stylesheets/desktop/topic-list.scss at main · discourse/discourse · GitHub
For the specfic change that cuased this, see UX: category images have no sizes (#6662) · discourse/discourse@1da265d · GitHub and UX: Use aspect ratio to calculate width and height precisely · discourse/discourse@5db50d6 · GitHub. The changes were made to prevent the page from “jumping” as images of unknown heights are loaded - we now reserve the space upon page load.