The OS X version looks ancient and not like a modern OS X app at all. Is that due to limitations in Qt? I had considered Qt for cross platform development, but after seeing that comparison it seems like it would be better to maintain separate front-ends for different platforms.
I am thinking of Pixelmator, or Apple's Final Cut Pro and Motion for comparison to your screenshot. Even modern versions of Photoshop manage to look much nicer than that.
It's got a custom overall stylesheet for the tab widgets as I don't like the OS X tab style or colour, so they're completely different.
Qt "emulates" native control appearances by drawing them itself. This means it doesn't get it 100% correct (in OS X's case). You can apply stylesheets to configure all aspects of spacing and appearance, so in theory it's possible to write a stylesheet to completely emulate OS X's controls (or at least the majority of them), but that'd be a lot of work which you probably wouldn't want to do.
I could make it any colour I wanted with stylesheets in Qt - this was a personal app, and I'm not too concerned with what it looks like.
I am thinking of Pixelmator, or Apple's Final Cut Pro and Motion for comparison to your screenshot. Even modern versions of Photoshop manage to look much nicer than that.