Evaluate whether SCSS can be replaced by vanilla CSS

[cjcenizal]

CSS has come a long way, e.g. introducing variables. We should audit our usage of SCSS, enumerate the features we're using, and evaluate them against their vanilla CSS alternatives. If there won't be any degradation in DX, we should migrate off vanilla SCSS entirely. This will simplify our build step, remove a dependency, and theoretically expand the pool of developers who will be familiar with our styling layer.