If I make use of the new styles available in Scrivener 3.0 (for macOS and the Windows 3.0 beta) - and then sync this project to an iOS device to further work on the project there, will the styles be preserved? Can I apply the 3.0 styles I’ve defined under iOS? Can new styles be created on iOS?
Yes, styles are preserved. Yes, you can apply styles using iOS Scriv: Use the paint brush icon to style text.
I’m not sure, but I don’t think you can create new custom styles on iOS.