Mac is always tricky, I don't know the answer in that scenario. In general, default styles should be available everywhere (you can run Windows style on Linux, for example). This is usually restricted to "base" styles (so, default Windows instead of Win7 style, for example).
Or you can go for QSS (see "this brilliant example":https://github.com/ColinDuquesnoy/QDarkStyleSheet), but be careful - QSS does introduce performance penalty that can be quite severe (app startup takes longer due to style parsing, for example).