If you use Qt it will use your system theme and colours no matter which OS you run your app on, unless you (the dev) overrides the style manually. I'd believe that's the case for wxWidgets too.
It's not about Qt's ability to pick up system theme & colors (although not also that the configurability mentioned in TFA goes somewhat beyond what is typically offered by modern desktop environments).
It's about whether or not the application uses custom elements (or even just puts everything in a Canvas and draws everything itself).
It's about whether or not the application uses custom elements (or even just puts everything in a Canvas and draws everything itself).