I've been reading some ideas around Custom Themes with style sheets and stuff. However, I have not seen ideas leveraging a JSON file. Power BI already allows the import of a JSON file to define a custom theme. It would be very useful if we could upload the same JSON file to PowerApps and that way keep consistency among Power BI and PowerApps, especially when embedding one into the other.
We are planning on both improving our default themes and allowing users to create custom themes.
Our first click stop is allowing users to customize colors in custom themes by June 2020.