I wrote about that once: https://codepen.io/chriscoyier/post/codepens-css
It hasn't changed too much. SCSS. Autoprefixer. Rails Asset Pipeline.
One thing that's changed since I wrote that is that more people are writing CSS around CodePen. I think the system has scaled fairly well. We try to name things intelligently, but don't stick to a particular strict naming convention. We try and not nest too much. We try to use handy Sass abstractions when they make the code easier instead of harder.
Our brand new Project Editor is in React and it even has (gasp) a sprinkling of inline styles.
I don't forsee any dramatic new changes in how we do CSS coming soon.
One thing I do see changing is possibly changing how we compile in development. I could see us switching to an all-Webpack thing and changing deployment commensurately. We used to have a Guard-based file watcher thingy so we could get live reload going on development, but that's kinda deteriorated and I, for one, really want my hot reloading back ;)