::theme & ::part support

https://meowni.ca/posts/part-theme-explainer/

I was wondering what we could do with this in Sheetify. The spec isn't done yet, but it's probably worth thinking about. I'm thinking there could be some interesting things we could do here.

cc/ @hughsk if you have any thoughts, that'd be amazing - but like no pressure, I know you're super busy haha.