Hey folks, good timing to surface this topic! 
We’ve been preparing for some time now to support all of you as marketplace partners to get your apps ready for dark mode. I’ve just shared an announcement about this: Introducing design tokens, new colour foundations and dark mode
I want to highlight (cross-posting from that announcement):
Design tokens will be in all Atlassian Design System components, as well as throughout front-end platform components in
@atlaskit, such as the Editor.
We, the Atlassian Design System team, have been working closely with our colleagues across teams that are responsible for different components that make up @atlaskit, as well as Jira Cloud teams on implementing design tokens and dark mode.
Specifically on the challenge of styled-components and Emotion, I hear you on that – moving off SC to Emotion is an ongoing effort that’s happening in parallel. Many @atlaskit components have been migrated already, and among the remaining ones I’m happy to share that work is nearing completion on migrating Editor components at the moment!
Check out that post for more details, and let me know your questions in that thread!