Not sure if you would like to modernize the code in general. Some methods still use that = this (instead of arrow functions) and could use async (e.g. cacheTheme).

Sorry, I missed to explain that:
The motivation of this refactoring is to extract code from the monolithic index.js script to improve testability of
individual functionalities. But I only focused on parts that are likely to be touched as part of #168 ("Improved CSS Variables support"). This PR is based on refactoring that was already made as part of that PR.

Using new language features is a good thing but I didn't focus on changing all the code. Can be done as another follow-up, along with fixing the max-len issues.
I only focused on using async/await as it really improves the code structure.

Rebased on top of master to also test on Node 16 (see #183)

Thanks 👍🏻

Note: I'm using rebase and merge to keep the history and have separate commits for changing the code and moving it to other files.