7.css, a very similar project, provides a scoped css file: https://khang-nd.github.io/7.css/#scoping
With this, only the selected UI is affected, e.g. not all buttons on the entire website.
The 'magic' happens in the build file and should be easy to replicate,
As this repo contains two themes, two scoped files and tags necessary, something like winxp and win98 would be great.