I'm not sure if this is an appropriate question for this site, but I tried to find an answer through Google and I found a question here about copying UI themes/paradigms so I hope it's not off-topic.
The answer to that question was that it's not illegal to copy as long as you don't use the exact same graphics.
But what if I want to implement a cross-platform UI framework, like Qt, wxWidgets or Java Swing and want to have a native look and feel on each platform? Would I have to obtain a license from Microsoft/Apple/etc. to create native-looking themes for this hypothetical framework? If so, does that mean that the aforementioned frameworks obtained such a license? What about UI mockup software like this?
I assume it wouldn't make any difference if this hypothetical framework is released under a very permissive license (MIT, BSD, zlib, ...) or as a commercial product?