cirrus, Went down a rabbit hole today trying to share some CSS between a document (i.e. "light DOM") and the shadow DOMs of some #HTML custom elements (aka #WebComponents).
This article by Eisenberg Effect was super useful: https://eisenbergeffect.medium.com/
I ended up building something along similar lines, but customised for my specific needs. Needs a bit of polish, but it works.
It really shouldn't be that hard to do though. Really hope a native way to do this ends up in the #web platform someday.