I'm having exactly the same issue trying to migrate to Emotion 10.
I built a UI library with Emotion 9 that is used by two Gatsby sites. The library was compiled with Rollup and additionally exported a theme object which both sites imported for their ThemeProviders.
Now with Emotion 10 the theme property received by the library components is empty. I've tried different combinations of emotion babel plugins/presets in my Rollup config as well as gatsby-plugin-compile-es6-packages but without success.
I would really appreciate some advice on how to implement an emotion-based ui library that uses themes.
@Andarist was basically on the money. While I had @emotion/core as a peerDependency I also had it as a devDepdendency to overcome the limitations of yarn/npm linking. I moved to a yarn workspace setup and removed the devDepdendency which solved my problem. @chukkwagon you should be OK with a monorepo.