Native web-components - styling shadow dom - how?

Hey,

i am working on native components. Well, edge and ie seems to have many problems but browser like chrome and firefox can handle it. Looks like.

I have an index.html file that contains css classes. I would like that this classes or at least some of them work in the shadow dom of my component.

I found a way for chrome to use ::part(). With this i can give styles to my Component "share-button" - share-button::part(button) {}

Well. In chrome and opera it works very well but not in firefox (latest version).

Is there a way to ship around for browser who do not support ::part()?

Thank you