There's already https://github.com/picoe/Eto if you want to use native controls - if that's what you need go for it! But there's a need for both approaches IMO and hopefully Avalonia can start to fill one of those!

EDIT: Ah you edited your question: yes controls are fully styleable - the reason we've got the "modern" look so far is simply because it's easy! More themes, and "native" looking themes are definitely welcome.