@FunkyBob@chaos.social I've done this in the past for some personal projects, though possibly that was a smaller scale than you're looking for.

It mostly involved 'converging' the styles first (identifying similar things and combining them into a set of shared semantic rules that's smaller and less duplicated in total), and then once that was done, changing the implementation of those rules.

The convergence step served mainly to prevent "oops I overlooked this one place that only partly shares this set of rules and now I accidentally broke it" by making sure that semantically similar elements actually use a similar set of class names.

The process is not that different from how I do this for other code refactoring, though in this case 'similarity' is determined by an element's purpose in the layout or design and visual style, rather than the logic of a function.