Flutter team member here. Yes you can! In fact, Flutter's design is layered in order to allow developers to customize it at whatever layer works for them. So in addition to being able to use Cupertino widgets out of the box (as referenced in another reply), you can also design your own widget look-and-feel if needed by extending the "widgets" library.
Yes! Flutter already supports iOS design with Cupertino widgets. It’s meant to be customizable so I’m sure other themes exist.
Wow nice, I didn’t know Cupertino widgets exist[0]. I guess we need them for Windows and Mac so it looks and feels native, I need to try that. Thanks