what's the legal status of copying an existing OS theme to implement a GUI framework

Question

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?

Answer 1

If your framework just calls the native UI-drawing methods for each element, then you'll automatically get the native elements, of course, and you won't be bundling any graphics into your own code, so there's nothing to license.

If you want to copy the UI elements directly into your project's files, that would require a license.

Answer 2

Each platform contains the same basic elements - draw window, draw button, etc. Each draw references would draw what the OS has defined as that object. I don't understand how you would implement a special UI-mimicking theme when it's all already built in.